126 
JOUENAL OF THE PLYMOUTn INSTITUTION. 
their passage through media of different density were treated of, 
and refraction as it takes place in the human eye. The various 
hindrances to perfect refraction were enumerated — chromatic and 
spherical aberration, and how they were corrected. There are, 
however, other causes that may cause imperfection of refraction, 
such as myopia, hypermetropia, presbyopia, and astigmatism. 
These were each described ; the troubles of hypermetropia and the 
great danger of short-sightedness were also mentioned. In order 
that objects should be perfectly seen that are placed at varying 
distances from the eyes, it is necessary that some integral change 
should take place in the organs themselves. This change is known 
as accommodation. Its performance was described and shewn to 
reside in the lens. The image of external objects being supposed 
to impinge on the retina perfectly refracted, the lecturer went 
on to demonstrate that the percipient element in the retina was the 
cones in the bacillary layer, and quoted instances of various 
analogues in the nerves of the other special senses. He also 
shewed in how great a measure vision was dependent upon the 
sense of touch for the perfection of ideas of external objects. He 
described the inversion of the image on the retina, but also shewed 
it was not necessarily so transferred to the brain. He regarded the 
reception of ideas from without as placing the brain in a state of 
consciousness to receive such ideas, but that they were not im- 
mediately transferred as such from the retina. 
Stereoscopic vision was treated of, and various other subjective 
phenomena, such as the ideas of size and distance, the retention of 
retinal impressions, complimentary colours, diffusion of retinal 
impressions, &c., and in conclusion he shewed that the difficulty of 
understanding vision lay in the space between the eyes and the 
brain. 
