ON INSTRUMENTS AND THEIE USE. 19 



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 membrane lining the choroid tunic, inside 

 the sclerotica. 



Now, if we make a section of the retina, and 

 examine it under the microscope we shall find its 

 structure to be as follows. 



Starting from the junction of the retina with the 

 vitreous humour, we have : 



(1) The layer of nerve fibres. 



(2) The layer of nerve cells. 



(3) The granular layer. 



(4) The inner granular layer. 



(5) The intermediate layer. 



(6) The outer granular layer. 



(7) A second fine membrane. 



(8) The layer of rods and cones. 



(9) Pigmentum nigrum of the choroid. 



The retina is the terminal organ of vision, all the 

 apparatus in front of it being merely for the 

 purpose of securing that an accurate image shall 

 be focussed upon it. As to how the luminous im- 

 pressions yield to us such a definite idea of things 

 is a question still under consideration ; many have 

 tried to solve it, but it is open to doubt whether we 

 are any nearer the mark than those philosophers 

 who lived 2,000 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 



