654 Proceedings of the Royal Society 
the temperature alone was raised to 34‘2 0 C,, 197 c.c. transuded in 
the same time. 
At tlie beginning of this paper I referred to Ludwig’s theory regard- 
ing 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 solu- 
tion. 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 cir- 
culating 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-. MTCendrick, M. I). 
There are several well-known methods of mixing colours, such as 
the superposition of two spectra or of different parts of the same 
