132 JOURNAL OF THE PLYMOUTH INSTITUTION. 
by the thickness of the retina no less than ten different layers 
were found, and the lecturer described the optic nerve, its fibres, 
and the method of their position, showing that they were not the 
recipient layers. Gradually excluding all other layers, he came to 
the layers of rods and bulbs, which he showed from the position to 
be the true visual layers of the organ. The other layers were 
briefly described, but only to show that they could not be the true 
recipient layers. He then proceeded to explain how the rays of 
light entering the eye first impinged on the posterior layer of rods 
and bulbs, passing through the other anterior ones. The struc- 
ture of the macula lutea, or yellow spot, was accurately described, 
and its differences from other regions pointed out. The theories of 
Max Schultze as regarded the action of the retina were spoken of 
at some length, as was also the comparative anatomy of some of the 
component elements. The analogy between the retina and the 
other organs of special sense was referred to, and especially as 
regarded the rods and bulbs. The subject of appreciation of colour 
was opened up. It was shown that in some animals globules of 
oil were placed in the cones, of different colours. It was supposed, 
according to the law of Helmholtz, that the nerve fibres in the 
coloured cones would only vibrate synchronously with rays of light 
of the same colour as in the organ of Corti, that certain nerves only 
vibrated synchronously with certain sounds. This hypothesis had 
been, however, broached years ago — in 1820 — by Dr. Thomas 
Young. The phenomenon of colour blindness was spoken of, and a 
hope was expressed that through this disease further investigations 
as regarded the physiology of vision might prove useful. The 
lecturer spoke at great length on the subject of alteration of im- 
pressions, especially during their passage through the layers of rods 
and bulbs, viz., of light into nerve force. The retention of retinal 
impressions was spoken of, and the theory of complementary colours 
gone into, Mr. Square regretting that the facts at hand were so 
few. The laws of Helmholtz on the subject were criticised, and 
generally agreed with. Many other phenomena, as the heightened 
effect of colours by contrast, the duration of impressions, and the 
habit of observation, were touched upon. Stereoscopic vision was 
explained, as well as the use of many instruments, such as the 
zooscope, stereoscope, ophthalmoscope, &c. Mr. Square concluded 
with an inquiry into the theory of inversion of the retinal image. 
