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was found that when water is passed into the vessels it almost immediately passes through the walls and causes oedema of the tissue, and the onward flow of the fluid is prevented. The salt solution seems, however, not to pass through the walls of the vessels into the lymphatic spaces so readily if the kidney is quite fresh, but still it passes from the glomeruli into the dilated end of the tubuli uriniferi. I found it very difficult to get this experiment to work satisfactorily, as the kidney requires to be used imme diately after the death of the animal, and a number of precautions need to be taken which it is not necessary to mention here.

In the experiments alluded to the salt solution was passed into the artery under various pressures, the venous resistance being equal to 20 mm. in all except the first, in which case no resistance was offered to the exit of the fluid by the vein. In the first experiment the solution seemed simply to pass from the arterial into the venous system, very little being pressed into the urine tubules. When, however, the efferent resistance is raised to 20 mm., and at the same time the afferent pressure advanced to 40 mm., the increase in the amount of fluid pressed into the ureter is obvious. In the other experiments upon the kidneys of animals, the results of which I will not give in detail, a somewhat similar plan was adopted. The following are the results:-(1.) When the fluid contained in the ureter is subjected to pressure, the quantity of fluid that passes from the vein is diminished in relation to the pressure employed, and so also is the amount of fluid that transudes from the glomerulus into the capsule of the Malpighian body. (2.) The quantity of fluid that passes from the vein depends upon the amount of afferent pressure; the greatest increase takes place between 40 and 50 mm. (3.) The temperature of the fluid affects the rapidity of the flow through the vessels and the quantity that transudes into the tubuli uriniferi. The higher the temperature the greater is the amount of fluid passed from the ureter, and the more rapid the circulation through the vessels. (4.) When the fluid is pressed into the artery, it finds its way readily into the vein, but when injected into the vein, it does not escape by the artery. There must, therefore, be some arrangement in the kidney, probably in the Malpighian body, by which regurgitation of the fluid is prevented.

The results of the experiments with the bowel show (1.) that

the amount of fluid that transudes is in accordance with the pressure upon the fluid inside. (2.) For every 10 mm. increase in the afferent pressure 275 c.c. more fluid transudes per minute, and the flow along the bowel is increased; whereas, when the efferent resistance is increased, the amount of fluid that transudes is augmented by 31 c.c. in the same time, and the flow along the bowel is diminished. Therefore the afferent pressure may be said to be expended in two ways-increasing the amount of fluid that transudes, and the quantity that passes along the bowel-but the efferent resistance exerts its whole force in pressing the fluid through the membrane, therefore, 10 mm. increase in the efferent resistance has more effect than the same increase in the afferent pressure, and for the same reason we would suppose that a given increase in the venous resistance would conduce more to rapid secretion of urine than the same increase in the arterial pressure, unless when the venous resistance is extreme when other factors come into play. (3.) The addition of urea slightly retards the transudation of water through the membrane. The filtrate contains the same percentage, whatever pressure may be employed, as the original solution. (4.) Albumen also retards the transudation of water, but it differs from urea in this respect, that the percentage of albumen in the filtrate is in relation to the pressure. The higher the pressure the larger the quantity of albumen in a given amount of the filtrate. (5.) The presence of urea in a solution of albumen assists the filtration of the albumen at the expense of the urea. The following table shows the results:

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(6.) The higher the temperature of the solution the more rapid the transudation of the fluid. Thus, when water was passed into the bowel at a temperature of 15.9° C., and under a pressure of 45 mm., 1425 c.c. filtered through in thirty minutes; whilst, when

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VOL. IX.

the temperature alone was raised to 34.2° C., 197 c.c. transuded in the same time.

At the beginning of this paper I referred to Ludwig's theory regarding the secretion of urine, namely, that the blood is subjected to a high pressure inside the glomeruli, a free filtration into the dilated end of the tubuli uriniferi takes place, and this filtrate, which is at first very dilute, gradually parts with a portion of the water that holds it in solution. This is believed to take place by a process of diffusion between the fluid in the tubuli uriniferi and the blood in the veins surrounding them on all sides. Now, if it were not that albumen retards to a certain extent the passage of crystalloids (salts and urea) through an animal membrane, then the fluid in the inside of the urine tubules would be of the same concentration (in crystalloids) as the blood. But it has been shown that when a solution of albumen and urea are filtered through an animal membrane under pressure, the filtrate is less concentrated than the original solution, particularly as regards the amount of albumen, but also to a slight extent the urea. This is more especially the case when the pressure is not great. If the blood contained nothing but crystalloids (urea and salts) then the fluid inside the tubuli uriniferi would be the same as the fluid circulating in the vessels, and no diffusion would take place during the passage of the urine from the glomerulus to the pelvis of the kidney. This is, however, not the case; the blood circulating in the vessels contains a large quantity of albumen, and, if the theory above stated be correct, more urea than the fluid in the tubules, so that, putting aside any special function the epithelium may have, diffusion must result, and a portion of the water in the tubules pass back again into the blood. This diffusion will take place as the urine passes along the tubuli uriniferi either till it becomes of the same concentration as the blood outside, or makes its escape into the common ducts that convey it to the pelvis of the kidney. Therefore, the longer the urine remains in the tubuli uriniferi, other things being equal, the more concentrated will it be.

4. Note of a Method of Studying the Binocular Vision
of Colour. By John G. M'Kendrick, M.D.

There are several well-known methods of mixing colours, such as the superposition of two spectra or of different parts of the same

spectrum-the method of reflection, Czermak's modification of Scheiner's experiment, the use of rotating disks having coloured sectors, and the direct mixture of coloured powders or coloured liquids. In all of these cases the effect may be seen with one eye, and is due to the action of light on a definite portion of one retina. But may sensations of mixed colours be produced by binocular vision of the components? Regarding this question various wellknown observers have arrived at completely opposite results. Thus, as mentioned by Helmholtz in his "Optique Physiologique," p. 976, H. Meyer, Volkmann, Meissner, Funke, and he himself fail in obtaining the sensation of the resulting colour, whilst Dove, Regnault, Brücke, Ludwig, Panum, and Hering state the reverse. In his great work, Helmholtz describes various methods by which he investigated the question, and his opinion amounts to this, that we have no true binocular perception of colour. According to him we may have a resultant sensation of a particular kind, different from that of the two components, but also unlike the sensation of the mixed colour obtained by methods appealing to one eye only.

In studying this subject I lately devised the following simple. arrangement: Take two No. 3 eye-pieces of Hartnack's microscope, or any similar eye-pieces of considerable focal length, and place one before each eye. If they be somewhat diverged, two luminous fields will be seen, and by adjustment, the edge of the one luminous field may be caused to touch the edge of the other. In these circumstances a definite area of each retina is illuminated. By converging the eye-pieces, the two fields may then be partially overlapped, and when the axes of the two eye-pieces are parallel, both fields coincide. It will then be found that the overlapped portion is intensely luminous, whilst the other portions become less luminous, as if cast into shadow. By increasing or diminishing the amount of convergence of the eye-pieces, the extent of the luminous field may be varied at pleasure, and the two fields coincide when the two images fall on the two yellow spots. If, then, a small piece of coloured glass be inserted into each eye-piece, say red into one and blue into the other, on repeating the experiment as above mentioned, I find that the overlapping portion of the two fields gives a sensation of the resultant colour. I have repeated the experiment with various coloured media, such as coloured gelatine paper, coloured

paper rendered translucent by oil, &c. In showing the experiment

people do not see the resultant The cause of this and of the

to others, I have found that certain eolour, whilst others do so readily. opposite statements of the observers above alluded to, I believe to be this: The sensation resulting from the fusion in the brain of the two impressions, one coming from each eye, appears to be capable of decomposition by a mental effort. Thus, the purple produced eye so long as I simply look

by red and blue appears as such to my at it without any conscious effort; but if I wish to analyse it, I then find that the two colours, red and blue, seem to be superposed on each other, and the one appears to shine through the other. On ceasing to make any effort, they again fuse together as before. Again, by thinking of the colour opposite the right eye, say red, the field ceases to be purple and has a decided tinge of red, and on thinking of the colour before the left eye, say blue, the prevailing tone of the field is blue. Apparently, then, if corresponding points of two retina be simultaneously stimulated by two different colours, the impressions are fused in consciousness into the resultant colour; but the resulting sensation may be decomposed by an act of attention. The decomposition is effected partly by strongly directing the attention to one eye, and less strongly to the other, and the result is a sensation corresponding to the colour placed before the eye to which the attention is most strongly directed. Some of the same facts may be studied with the aid of the stereoscope.

The following Gentlemen were duly elected Fellows of the Society :

ALEX. MACFARLANE, M. A., B.Sc., 2 Roseneath Terrace.
SAMUEL DREW, M.D., D.Sc., Chapelton, near Sheffield.

GEORGE M'GOWAN, 24 Argyll Place.

JAMES BRUNLEES, Vice-Pres., Inst. C. E., 5 Victoria Street, Westminster.
JOHN GRAHAME DALZIEL, 95 South Street, St Andrews.

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