THE ELEMENTS OF MICROSCOPY.—THE HUMAN EYE. 297 
The nerve fibres of the retina are excited probably by a product 
of the action of the light picture upon the visual purple, and the 
irritations are transmitted to the brain by the optic nerve, produc- 
ing the sensation of vision. 
The picture produced upon the retina has been compared with 
that produced by a photographic lens upon a screen or ground 
glass ; but it will be seen that the instances are not strictly parallel. 
In the eye the rays falling upon the cornea do not again en- 
counter air, the picture is formed i the highly refractive substance, 
while in the photographic image air intervenes between the screen 
and the lens, and between the lenses themselves. 
Then, again, the adaptation of the eye to various distances is 
obtained by a process so dissimilar to that of the lens in the camera, 
that it is well no comparison should be instituted. 
The retina has been previously described as a delicate mem- 
brane lining the choroid tunic, inside the sclerotica. 
Now, if we make a section of the retina we shall probably 
find its structure very similar to the diagram. (Fig. 26.) I say 
probably, as I have never met with sections which displayed the 
structure so well as Max Schultze has indicated. He has described 
the various layers which you see before you, as follows :— 
Starting from the junction of the retina with the vitreous humour, 
we have— 

The layer of nerve fibres ............... a. | The outer granular layer.............. 
The layer of nerve cells.................. b, A second fine membrane ............ ff. 
The granular layer ..........:sccsieessse0 ¢. | The layer of rods and cones ......... h. 
The inner granular layer ............... @. | Pigmentum nigrum of the choroid...7. 
The intermediate layer .................- é. 
The retina is the terminal 
organ of vision, all the ap- 
paratus in front of it being BF 
merely for the purpose of fi 
securing that an accurate Baa 
image should be focussed 
upon it. As to how the 
luminous impressions yield 
to us such a definite idea 
of things is a question still 
under consideration, many have tried to solve it, but Iam not sure 
whether we are any nearer the mark than those philosophers who 
lived 2000 years ago. 
There are several curious properties inherent in the retina. By 
means of the ophthalmoscope may be seen a point, a little out 
of the centre, where the optic nerve enters the eye. This spot is 
totally blind, it cannot perceive a trace of light, and if the image 
of an object falls upon this blind spot, that object is totally in- 


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