Flowthrough Cuvette

Abstract

The invention relates to optical cuvettes of the throughflow type and to the production thereof. The cuvette comprises a stratified block composed of at least three layers which together define a space constituting a doubly-bent continuous passage with a central elongated cavity communicating with two transverse branch-channels extending to the cuvette surface. An intermediate layer consists of a pair of transparent plates having oppositely arranged inner edges which are shaped in accordance with the path of said passage and are rounded off between the central cavity and the transverse channels, to provide streamline flow in the passage. Two lateral layers consist of plates which respectively close off the passage on either side thereof, between the shaped inner edges of said pair. The invention further provides a manufacturing method wherein two plate-edges are shaped in accordance with the profile of the doubly-bent passage and are oppositely arranged at a given distance to form an intermediate layer on to which two lateral plates are then mounted to close off the passage on either side thereof.

Description

There are various types of optical cuvettes commonly used for holding a liquid specimen or sample intended to undergo optical measurement, for example by means of a spectrophotometer. To this end, the known optical cuvettes generally consists of a transparent body made for instance of glass or quartz and comprising a cylindrical cavity for receiving the specimen. This cavity extends inside the cuvette over a given distance constituting the "optical path" travelled by the light beam passing through the sample during the optical measuring operation. The two ends of the cavity are delimited by transparent zones of the cuvette wall, which zones constitute inlet and outlet windows for the said beam.

This cavity communicates with the outside of the cuvette via two transverse channels which are respectively connected to the two ends of the cavity and enable the sample to be inserted in the cavity and to be removed therefrom after the measurement has been effected. The cylindrical cavity thus forms a central elongated limb of a doubly-bent continuous passage.

One type of commonly used optical cuvette consists of a quartz or glass tube, which is bent at two places to from the said passage. The central cavity of this passage is thus delimited at its ends by curved portions of the wall of the twice-elbowed tube forming the cuvette. Consequently, the said inlet and outlet windows for the measuring beam are also curved, thus causing light deviation, and hence a risk of notably affecting the accuracy of the optical measurement. Further, this mode of production, which consists in bending a tube, does not make it possible to determine in a very accurate reproducible manner the length of the said optical path for the beam of light passing through the liquid specimen. Now, even a relatively small variation of this length, between different cuvettes, is liable to cause notable measurement errors.

In a variant of the above-described known optical cuvette, the said windows are flattened to obviate the cited drawbacks. To this end, the twice-elbowed tube is softened by heating and two flat plates are pressed against the curved windows to flatten them and to adjust the distance between them. Such indirect deformation of the internal surface of the windows, by acting on their outside faces, does not however make it possible fully to obviate the above-mentioned drawbacks of the type of cuvette under consideration.

In a second variant of this type of known cuvette, the twice-elbowed tube is cut out so as to remove the said curved portions defining the two ends of the cavity and two flat transparent plates are fixed in their stead so as to form flat windows. This cutting operation obviously requires a very high degree of accuracy so as to set in a precise manner the distance between the said plates and hence the optical path of the beam. The cuvette according to this second variant does nonetheless enable the above-mentioned drawbacks to be obviated to a large extent, provided this accuracy is ensured. However, it has another drawback due to the fact that the said plates are disposed transversely to the central cavity and thus form corners, approximately at right angles, at the two ends of this cavity, such corners forming stagnant zones in which a residue of the liquid tends to remain after discharge of the sample. Such a residue from one sample to the next can however notably affect the accuracy of the optical measurement.

Besides the above-mentioned drawbacks, the production of this type of known cuvette, as well as of the two described variants, gives rise to problems which become more and more difficult to solve as the size of the cuvette decreases. Thus, it has not been possible so far to produce cuvettes as described having the very small dimensions needed to perform optical measurements on liquid micro-specimens.

Another known cuvette, which is intended for micro-samples, comprises a block traversed by a capillary bore forming the central measuring cavity and transparent plaquettes mounted at the ends of this cavity to form the previously mentioned windows for the entry and exit of the light beam serving to effect the optical measurement. Two lateral bores are moreover provided in the block so as to connect the two ends of said central cavity with the exterior and to thus form a doubly-bent passage with the central bore. This type of capillary cuvette allows the length of the optical path of the light beam traversing the cuvette to be fixed quite accurately. However, the doubly-bent passage, delimited by the walls of said bores, likewise has right-angled corners and sudden changes of direction at both ends of the bore forming the central cavity. This results in the previously mentioned drawback of stagnanant zones leading to sample residues which can render the measurements inaccurate from one sample to the next.

A different type of known cuvette is the so-called "thin-layer cuvette," wherein the liquid is admitted between two transparent plates mounted at a slight distance from each other and forming the said windows for entry and exit of the light beam. The use of this type of cuvette is however limited to effecting measurements on opaque liquids, such as non-diluted blood, due to the fact that the optic path through the thin liquid layer is very short.

The object of the present invention is to produce an optical cuvette of the throughflow type, which is relatively easy to manufacture in a reproducible manner and which, in addition, enables the above-mentioned drawbacks of the known cuvettes to be obviated.

The invention provides an optical cuvette of the throughflow type having therein at least one double-bent continuous passage composed of a central elongated cavity for enclosing a liquid sample and of two transverse branch-channels extending between the ends of the cavity and the cuvette surface to allow admission and discharge of said sample, the cuvette being at least partially transparent so as to allow a light beam for effecting optical measurement to pass through the cuvette and thereby to longitudinally traverse the elongated cavity containing the sample.

The cuvette according to the invention comprises a stratified block composed of at least three layers arranged in juxtaposition so as to define together a space constituting the said passage, an intermediate layer of said three layers being composed of a pair of transparent plates lying in a common plane at a predetermined distance from each other, each plate of said pair being provided with an inner edge which is shaped in accordance with the doubly-bent path of said passage and which has a gradual transition between said cavity and said transverse channels whereby to allow streamline flow along said passage, and two lateral layers being formed by two corresponding plates whereof the inner surfaces respectively define said passage on either side thereof, between said shaped inner edges of said pair of plates of the intermediate layer.

The invention also provides a method of manufacturing an optical cuvette of the throughflow type having at least one doubly-bent continuous passage composed of a central elongated cavity and of transverse branch-channels respectively extending between the ends of the cavity and the cuvette surface, the cuvette being at least partially transparent so as to allow a light beam to pass through the cuvette and thereby to longitudinally traverse the elongated cavity, said method comprising:

shaping at least one transparent plate so as to obtain at least one pair of edges corresponding to the desired double-bent path of said passage and providing a gradual transition between said central cavity and said transverse channels;

arranging said edges in spaced relationship facing each other to form a first pair of wall portions partially defining said passage so as to provide central cavity of predetermined length;

and sealingly mounting two lateral plates so as to close-off either side of the space lying between said spaced edges, whereby to form a complementary pair of wall portions defining said passage together with said first pair of wall portions.

The accompanying drawings represent, schematically and by way of example, various optical cuvettes embodying the features of the invention and also illustrate a method of manufacturing an optical cuvette in accordance with the invention, as well as a variant of this method.

FIG. 1 shows in perspective, partly borken away, an optical cuvette according to a first form of embodiment.

FIG. 2 is a front view of the cuvette according to FIG. 1, with an outer plate removed.

FIGS. 3a to 3d show, in section, four variants of the cuvette according to FIG. 1.

FIG. 4 respesents, in longitudinal section, a double cuvette according to a second form of embodiment.

FIGS. 5a to 5b schematically illustrate two steps of the manufacturing method.

FIGS. 6a and 6b schematically illustrate two steps of a variant of the manufacturing method.

FIGS. 7 and 8 respectively show a longitudinal and a transverse sectional view of a third form of embodiment.

The cuvette represented in FIGS. 1 and 2 consists of a stratified block 1, made up of three layers. Two small rectangular plates 2a and 2b form the outer layers of this block 1 and a pair of similar plates 3a and 3b having the general form of the letter L are arranged in spaced head to foot relationship in the plane so as together to form its intermediate layer.

As will be seen in FIG. 1, an elongated cavity 4 extends inside the block 1 along the mean longitudinal axis 0-0 representing the optical path and the ends of this cavity respectively communicate with two channels 5 and 6 which extend on opposite sides of this axis 0-0 and at right angles to the latter, as far as the opposite faces A and B of the block 1 (see FIG. 2). As may moreover be seen in FIG. 1, this cavity 4 and these channels 5 and 6 thus constitute a continuous doubly-bent passage which extends through the block 1, in the intermediate layer of the latter.

This continuous passage 5 - 4 - 6 has a rectangular cross-section with a first pair of opposite sides respectively defined by the internal edges of the intermediate pair of plates 3a and 3b and a second pair of opposite sides respectively defined by the outer plates 2a and 2b. Thus, as will be observed from FIG. 1, the inner edges of the plates 3a and 3b comprise three consecutive surfaces 7a, 8a, 9a and 7b, 8b, 9b, respectively, which form pairs of opposite surfaces, i.e. 7a and 7b, 8a and 8b, 9a and 9b, which pairs define two opposite sides of the channel 5, of the cavity 4 and of the channel 6, respectively.

These surfaces 7a, 8a, 9a and 7b, 8b, 9b are machined and polished, before the plates 3a and 3b are mounted in the block 1, so as to properly round the angles respectively formed between the successive surfaces 7a and 8a, 8a and 9a, 7b and 8b, and 8b and 9b. This rounding of the said angles enables any abrupt deviation along the said doubly-bent passage 5 - 4 - 6 to be eliminated by producing a gradual transition between the cavity 4, on the one hand, and the channels 5 and 6, on the other hand, thus eliminating any stagnant zone during the flow of liquid through this double-bent passage.

It thus becomes possible completely to discharge from the described cuvette each liquid sample that has served for the desired optical measurement. This discharge can be done by inserting into the cuvette another liquid thereby to force all of the liquid of said sample out of the cavity 4, without leaving any residue thereof in this cavity. In most cases, this other liquid may be the liquid sample which is to be subjected to the next optical measurement.

The structure of the described cuvette, in the form of a stratified block made up of four plates 2a, 2b, 3a and 3b, thus enables the cavity 4, as well as the channels 5 and 6, to be defined by the surfaces 7a, 8a, 9a and 7b, 8b, 9b which are readily accessible and which can thus be machined and polished without difficulty before assembling the cuvette. This not only enables the said passage to be given an outline which is favourable for complete discharge of each sample, but also to delimit the cavity 4 at its ends by windows having a flat polished surface and arranged at a precise predetermined distance from each other.

The plates 3a and 3b forming the intermediate layer are made of a transparent material, e.g. glass, quartz, or a plastics material such as acrylic resin. This material is selected in dependence on the wavelength of the light intended to perform the required optical measurement, such that this material be transparent to the light having this wavelength. The side plates 2a and 2b are made of the same material as the intermediate plates 3a and 3b. Since the side plates 2a and 2b are not intended to be passed through by the light used to perform the optical measurement, in particular in a spectrophotometer, these plates are preferably blackened to reduce lateral light diffusion.

The assembly of the cuvette can be achieved in any suitable manner. In this instance, the plates 3a and 3b may be assembled with the side plates 2a and 2b by heat bonding causing localized fusion of the adjacent surfaces of these plates.

The described cuvette is particularly suitable for optical measurements in a spectrophotometer and the mean axis 0-0 will then coincide with the optical axis of the spectrophotometer light beam.

The described form of embodiment can obviously be modified in various ways, with a view to adapting it better to the particular use to which the cuvette is to be put in each case.

Thus, for example, the channels 5 and 6 may be ground conically, after assembly of the described cuvette, to facilitate connection of these channels to a feed tube and to a discharge tube for the samples. This conical grinding, illustrated in dash-dotted lines in FIG. 2, may form an angle of 5.degree. in relation to the respective axes of the channels 5 and 6.

Further, it is not necessary for the stratified block constituting the cuvette to be rectangular, as described. Thus, the side plates 2a, 2b and the intermediate plates 3a and 3b may have different outlines like those shown by way of example in FIGS. 3a to 3d. These latter figures illustrate moreover some variants for the said double-bent passage consisting of the cavity 4 and of the channels 5 and 6.

Thus, for example, in the variants illustrated in FIGS. 3c and 3d, the cavity 4 further comprises at least one zone, 10 and 11, 12 respectively, which is located at a greater distance from the mean axis 0-0. Microbubbles of gas, which are often present in liquid specimens, can thus rise in the cavity 4 and accumulate in such a zone, which is located outside the axial measurement zone as such. It thus become possible to obviate the errors due to the presence of these microbubbles in the specimen during the optical measuring operation.

FIG. 4 shows an optical cuvette of similar structure to that shown in FIGS. 1 and 2. However, the laminar block 1 is here made up of five layers and forms a double cuvette comprising two similar cavities 4 and 4', with their associated channels 5, 6 and 5', 6'. The block forming this double cuvette comprises plates 2a, 2b and 3a, 3b which define the cavity 4 and the channels 5 and 6, as already described in relation to FIGS. 1 to 3. To form the second cavity 4' and associated passages 5' and 6', this block moreover comprises a further pair of intermediate plates 3a' and 3b', similar to the plates 3a and 3b described above, as well as a further side plate 2c similar to the plates 2a and 2b. The central plate 2b thus serves to define at the same time a part of the passage 5 - 4 - 6 and of the similar passage 5' - 4' - 6'.

The cavities 4 and 4' of the described double cuvette are thus arranged in identical manner and can for example be used to respectively receive a liquid specimen intended to be subjected to an optical measurement and a standard specimen used for a comparative measurement. The described double cuvette can thus be used to advantage to carry out optical measurements in a spectrophotometer of the well-known double beam type.

FIGS. 5a and 5b illustrate schematically two operations of a method of manufacturing an optical cuvette such as the one in FIG. 1. As is apparent from FIG. 5a, a glass or quartz plate 3 is first cut by means of an ultrasonic cutting tool 13 which has an S-shaped outline corresponding to that of the desired passage 5 - 4 - 6 so as to obtain a pair of identically shaped half-plates each comprising an edge formed by the three surfaces 7a, 8a, 9a and 7b, 8b, 9b, respectively. As will be observed from FIG. 5b, several of these shaped plates are then placed side by side, and subjected to a polishing operation by means of a polishing tool 14.

The inner surfaces of the said windows through which the light beam is intended to pass are then rendered perfectly flat and smooth and these surfaces may moreover be very accurately situated in all of the plates thus shaped. dimensions

The cuvette is finally assembled as shown in FIG. 1 by juxtaposing in spaced head to foot relationship in the same plane, a pair of shaped and polished plates thus produced, e.g. in a jig having the initial dimensions of the plate 3, by disposing on opposite sides of this pair side plates 2a and 2b (see FIG. 1) and by finally heat bonding the contacting surfaces between each of the plates 2a and 2b, on the one hand, and the plates 3a and 3b, on the other hand.

FIGS. 6a and 6b illustrate a variant of the described method. As is apparent from FIG. 6a, use is also made of an ultrasonic cutting tool 17 having an outline in the form of an S corresponding to the desired profile of the passage 5 - 4 - 6. However, in this instance, the plate 3' is wider than the tool 17 so that this plate 3' remains in one piece after the cutting operation and comprises a central zone cut in the form of an S. As shown in FIG. 6b, a polishing tool 18 is then used to polish those parts of the surfaces 7b and 9a, which define the ends of the cavity 4, where the exit windows for entry and exit of the beam are to be located.

The plate 3', thus shaped and polished, is then assembled by fusion bonding, as already described, with two side plates of the same size to form a stratified block which is wider than the final cuvette-forming block. The longitudinal edges of the three plates forming this block are then cut, as indicated by dash-dotted lines in FIG. 6b, so as to clear the inlet and outlet openings of the channels 5 and 6 and to give to the stratified block the desired final dimensions of the cuvette. There is thus produced a cuvette similar to that illustrated in FIG. 1, but having a cavity 4 with a profile similar to that shown in FIG. 3c. This variant according to FIGS. 6a and 6b facilitates the assembly of the stratified cuvette-forming block by obviating the need for accurately spaced juxtaposition in head to foot relationship of the plates 3a and 3b forming the intermediate layer, so as to obtain the exact desired length of the cavity 4.

It should be noted that the methods described above by way of example lend themselves to the manufacture on a large scale of cuvettes according to the various forms of embodiment of the invention, whilst ensuring very high accuracy and obviating the previously mentioned drawbacks of the known cuvettes. Indeed, these method have enabled cuvettes according to FIG. 1 and to FIG. 4 to be manufactured very accurately and with very small dimensions, such as are needed to perform optical measurements on liquid microspecimens, the overall dimensiosn of these cuvettes being at most of the order of 1 to 2 cm.

The outline of the doubly-bent passage may obviously have various forms other than those described and may be obtained by other means than an ultrasonic cutting tool, as described above. This outline may thus be obtained by any suitable machining means or by moulding.

FIGS. 7 and 8 represent a further embodiment of a double cuvette comprising two cavities 4 and 4' having a profile similar to that of the variant according to FIG. 3c. This profile is defined by two pairs of plates 3a, 3b and 3a', 3b', which are respectively mounted between plates 2a, 2b and 2c (see FIG. 8).

However, as may be seen from FIGS. 7 and 8, the same pairs of plates are turned around by 180.degree. with respect to each other, around a vertical axis in the present case. An arrangement is thus obtained, wherein the transverse channels 5 and 5', as well as 6 and 6', respectively extend to diagonally opposite locations on the same sides of the cuvette. In view of the relatively small width of the cuvette, this arrangement provides the advantage of considerably increasing the spacing between the channels which open on to the same face of the cuvette and of thus facilitating connection of the channels 5, 5' and 6, 6' to outside conduits, as described below.

In this embodiment, the cuvette is further provided with a pair of connecting blocks 19a and 19b made of glass, of plastics material or of metal, for example. These identical blocks are respectively mounted on the opposite faces of the cuvette, on to which the transverse channels 5, 5' and 6, 6' open, this assembly being achieved by gluing, welding or mechanical fastening for example.

Each of the blocks 19a and 19b comprises two connecting channels, 20, 20' and 21, 21', respectively, each aligned with one of the transverse channels 5, 5' and 6, 6' and each having a cross-section which varies progressively from a rectangular section, corresponding to that of the opposite transverse channel, up to a circular section communicating with a conical cavity 22, 22' and 23, 23', respectively, leading to the external surface of the corresponding block 19a or 19b.

These blocks 19a and 19b serve for the connection of conduits 24, 24' and 25, 25', respectively, and allow admission and discharge of the liquid samples which are to undergo optical measurement, in the central cavities 4 and 4' of the cuvette.

In the present instance, each of the conduits 24, 24' and 25, 25' has an enlarged conical end portion sealingly mounted in the corresponding cavity 22, 22' and 23, 23', so that its internal section is situated so as to form a prolongation of the corresponding connection channel 20, 20' and 21, 21', respectively. These conduits are moreover mounted by means of shaped clamping plates 26a and 26b which are respectively fixed on to the opposite external faces of the blocks 19a and 19b. As may be seen from FIGS. 7 and 8, tightening springs 27, 27' and 28, 28', which are respectively mounted in recesses provided for this purpose in the plates 26a and 26b, each press against the enlarged end of one of the conduits 24, 24' and 25, 25', via a washer 29, 29' and 30, 30', respectively. These four conduits are thus connected in a simple manner to the descirbed double cuvette, via the connecting blocks 19a and 19b having the channels 20, 20' and 21, 21' which provide a gradual transition of the flow cross-section at the outer end of the transverse channels 5, 5' and 6, 6' respectively associated with the central cavities 4, 4' of the passages of the double cuvette.

The double cuvette described above and shown in FIGS. 7 and 8 is further equipped with a heating device adapted to maintain the cuvette, and hence the liquid contained therein, at a desired temperature. This device comprises an electrical resistance 31 coiled around the cuvette, together with its connecting blocks 19a and 19b, in a zone extending practically over the whole length of the central cavities 4 and 4' the cuvette. This resistance is supplied with current from an electrical source 32 (see FIG. 8) which is controlled in a well known manner by means of a thermostatic regulator (not shown) associated with a temperature measuring member 33 mounted in the middle of the median plate 2b situated between the two passages 5 - 4 - 6 and 5' - 4' - 6' of the described double cuvette.

Maintenance of the temperature of the samples at a constant value, by means of a heating device such as described above by way of example, is advantageous for various optical measurements, such as those intended for determining the activity of enzymes; in that case, the temperature may be maintained at 37.degree. C for example. Heating of the cuvette may moreover obviously be ensured by any other conventional means, such as a thermostatically controlled circuit of heating liquid, arranged in any suitable manner allowing the samples contained in the cuvette to be maintained at the desired temperature.

It is understood that all the other cuvettes which have been described may likewise be equipped with heating means such as are mentioned and are described above.

When the cuvette is not provided with heating means, it may nevertheless be useful to equip it with means for measuring the temperature of the samples in order to be able to take it into account when effecting the optical measurements.

As appears from the described embodiments and variants, the optical cuvette according to the invention is of the throughflow type comprising a traversing passage having a central elongated cavity through which a light beam is made to travel longitudinally for effecting optical measurement. Thanks to the particular structure of the cuvette in the form of a stratified block, it becomes possible to round off the inlet and the outlet of the said central cavity, at the places where the latter communicates with the transverse channels leading to the exterior of the cuvette. As a matter of fact, this rounding off may be readily carried out before assembling the cuvette, i.e. before mounting the lateral plates so as to complete the wall of the traversing passage on either side thereof. Thanks to this rounding off, it thus become possible to give the said passage a longitudinal profile which is such as to ensure optimum liquid flow conditions from which any dead zone is eliminated. One may thus ensure very effective sweeping of the central cavity and hence eliminate therefrom any residue of each specimen once it has undergone the optical measurement. This important advantage is not only limited to measurements which are carried out discontinuously on stationary samples, but also applies to continuous measurements which are effected on liquid specimens flowing continuously through the central cavity.

Besides the advantages mentioned above, it may be noted that the cuvette according to the invention does not have the previously mentioned limitations of conventional cuvettes of the same type. As a matter of fact, it may be readily manufactured very accurately and in large series, without its dimensions playing a critical role; the manufacture of so-called "capillary" cuvettes in accordance with the invention thus does not present any particular problem. In addition, the cuvettes may be utilized for effecting optical measurements on all sorts of liquid samples and, when the liquids in question are too opaque, they may be readily diluted in order to obtain more transparent specimens.

It is understood that the desired cuvettes may be used for all sorts of optical measurements which may be based, for example, on the absorption or the fluorescent properties of the specimens. In the latter case, the lateral layers of the cuvette should likewise be transparent, in order to allow measurement of the fluorescent properties of the sample, perpendicularly to the axis of the central cavity.

Claims

We claim:

1. An optical cuvette of the throughflow type having therein at least one doubly-bent continuous passage composed of a central elongated cavity for enclosing a liquid sample and of two transverse branch-channels respectively extending between the ends of the cavity and the cuvette surface to allow admission and discharge of said sample, the cuvette being at least partially transparent to allow a light beam for effecting optical measurement to pass through the cuvette and thereby to longitudinally traverse the elongated cavity containing the sample, wherein the cuvette comprises a stratified block composed of at least three layers arranged in juxtaposition so as to define together a space constituting the said passage, an intermediate layer of said three layers being composed of a pair of transparent plates lying in a common plane at a predetermined distance from each other, each plate of said pair being provided with an inner edge which is shaped in accordance with the doubly-bent path of said passage and which has a gradual transition between said cavity and said transverse channels whereby to allow streamline flow along said passage, and two lateral layers being formed by two corresponding plates whereof the inner surfaces respectively define said passage on either side thereof, between said shaped inner edges of said pair of plates of the intermediate layer.

2. An optical cuvette according to claim 1, wherein the transverse branch-channels extend in opposite directions from the ends of the central cavity so that the passage extends according to a generally S-shaped path opening on opposite sides of the stratified block.

3. An optical cuvette according to claim 2, wherein the inner edges of the pair of plates forming the intermediate layer are generally S-shaped and have a rounded-off edge-portion situated in the region of transition between the cavity and the transverse branch-channels, the plates of said pair being arranged in spaced head-to-tail relationship in the intermediate layer.

4. An optical cuvette according to claim 3, wherein at least one of the inner edges is so shaped as to provide at least one enlarged portion in said cavity.

5. A method of manufacturing an optical cuvette of the through-flow type having at least one doubly-bent continuous passage composed of a central elongated cavity and of two transverse branch-channels respectively extending between the ends of the cavity and the cuvette surface, the cuvette being at least partially transparent so as to allow a light beam to pass through the cuvette and thereby to longitudinally traverse the elongated cavity, said method comprising:

shaping at least one transparent plate so as to obtain at least one pair of edges corresonding to the desired double-bent path of said passage and providing a gradual transition between said central cavity and said transverse channels;

arranging said edges in spaced relationship facing each other to form a first pair of wall portions partially defining said passage so as to provide a central cavity of predetermined length;

and sealingly mounting two lateral plates so as to close-off either side of the space lying between said spaced edges, whereby to form a complementary pair of wall portions defining said passage together with said first pair of wall portions.

6. The method defined in claim 5, wherein said pair of edges are formed by cutting out of the transparent plate a space corresponding to the desired doubly-bent longitudinal profile of the passage, and rounding off said edges so as to provide a gradual transition in the bent portions thereof between the central cavity and the transverse branch-channels of said passage.

7. The method defined in claim 6, wherein the said space cut out of the transparent plate has a generally S-shaped profile and the edges defining said space are rounded off so that the transverse channels gradually merge into the central cavity of the passage.

8. An optical flow-through cuvette comprising a body means, said body means having at least one internal layer, said layer including a pair of transparent plates in a common plane and spaced from one another, the spacing of said plates providing three generally linear passages in successive communiccation with one another, the first and last passages being substantially parallel to one another and smoothly merging with the middle passage, the middle passage being located at an angle to said first and last parallel passages whereby fluids flowing through said passages will not cavitate and an accurate optical measurement may be taken.

9. A cuvette as in claim 8 wherein the merger of said first passage with said middle passage provides a curved surface which is a mirror image of the merger of said last passage with said middle passage.

10. A cuvette as in claim 8 wherein the sides of said passages in said transparent layer are equidistant from the centers thereof.

11. A cuvette as in claim 8 wherein said body means comprises two layers on opposite sides of said transparent layer.

12. A cuvette as in claim 8 wherein said middle passage is longer than said first and last passages.

13. A cuvette as in claim 8 wherein said middle passage has a central axis adapted to be horizontal when the cuvette is used, said middle passage having a top and bottom side in said transparent layer, the bottom side of said passage being parallel to said axis and the top side being inclined relative thereto.

14. A cuvette as in claim 13 wherein said middle passage is longer than said first and last passages.

15. An optical flow-through cuvette comprising a body consisting of three planar layers, the outer layers being non-transparent, the inner layer including a pair of identically configured transparent members, said members being juxtapositioned to form said inner layer and having three surfaces internal to said outer layers, said members being spaced from one another to form three generally linear passages in successive open communication with one another and opposite edges of said cuvette, the first and last passages being disposed at identical but opposite angles to the ends of said middle passages, said middle passage being adapted to be horizontal when a fluid is passed through said passages and an optical measurement can be taken linearly of said middle passage.

16. The cuvette of claim 15 wherein said first and last passages are disposed at right angles to said middle passages.

17. A cuvette as in claim 16 wherein said middle passage has a larger cross-sectional area than said first and last passages.

18. A cuvette as in claim 15 wherein the merger of said first and last passages with said middle passage having larger cross-sectional areas than said middle passage.

19. A cuvette as in claim 15 wherein said middle passage has a larger cross-sectional area at its intersection with said first passage than it does at its intersection with said last passage.

20. A cuvette as in claim 19 wherein said middle passage is adapted to be horizontal and defines an upper wall and a bottom wall, said bottom wall being parallel with the horizontal and said top wall diverging away from the horizontal from the last passage to said first passage.