The Evolution of Optics. 267 



phenomena, no doubt lie some of the most important 

 future discoveries, some of the greatest achievements of the 

 science of optics. 



In our consideration of physiological optics we shall not 

 formulate any theories or attempt any explanation of the 

 marvelous transformation of ethereal vibrations into light- 

 sensations. Such an explanation belongs rather within the 

 field of physiological psychology. "We will first take up and 

 briefly describe that organ in which the first step of this 

 wonderful metamorphosis is accomplished and by means 

 of which the ether wave-energy is converted into nerve 

 activity. 



In these days of amateur photography the comparison of 

 the eye to a photographic camera will enable almost every 

 one to form a clear conception of its structure. A camera 

 consists essentially of a box lined within with some dark 

 material, having on one side an opening into which are 

 fitted a series of lenses so arranged as to throw an image of 

 the desired object upon a sensitized plate, which is to receive 

 the picture, situated at the opposite side from the lenses. A 

 diaphragm or shutter to regulate the amount of light ad- 

 mitted to the camera and some arrangement for adjusting 

 its focus for both near and distant objects is also present. 

 Now, this is practically the same mechanism that we find in 

 the eye. Corresponding to the box, we have in the eye a 

 strong fibrous coat, the sclerotic, commonly called the white 

 of the eye, in the front of which is situated a transparent 

 portion known as the cornea. Back of this cornea hangs a 

 curtain, the iris, with a small circular opening in the center, 

 the pupil, which diminishes or increases automatically in 

 response to more or less light-stimulation. This arrange- 

 ment is approximated in some of the most improved pho- 

 tographic cameras by what is known as an iris diaphragm ; 

 but this has not yet been sufficiently perfected to work 

 automatically. Back of the iris is a lens of such power that 

 in a normal eye rays of light are focused upon the back- 

 ground of the eye, where is situated that structure which 

 corresponds to the sensitized plate. This is known as the 

 retina, which we may consider for the present to consist of 

 the delicate terminal filaments of the optic nerve that, curi- 

 ously enough, spreads itself out on the front of the retina 

 and turns its sensitive tips away from the light back into 

 the retina. A layer of pigment gives the eye its black lin- 

 ing, and the greater part of the interior is occupied by a 



