794 PHYSIOLOGY. 



SPHERICAL ABERRATION, which interferes with distinctness, is 

 gotten rid of by cutting off outside rays. In the camera this point 

 is accomplished by the insertion of a diaphragm through a slit in 

 the lens-tube. The diaphragm is pierced by holes a larger or 

 smaller one being used according as the light is feeble or strong. 



The eye may be very aptly compared to the camera. It has a 

 small opening in front through which pass the rays of light. The 

 sclerotic and choroid coats form its walls. The refracting lenses are 

 the cornea, aqueous humor, crystalline lens, and vitreous humor. 

 They all tend toward the accomplishment of the same end : to bring 

 parallel rays of light to a focus upon a sensitive plate (the retina), 



Fig. 341. Diagram Illustrating Spherical Aberrations. (GANOT.) 



The rays passing through the edge of the lens have a shorter focal 

 distance than those passing nearer to the center. 



there to form a real inverted image of the object. Last, the iris with 

 its pupil acts as a diaphragm. 



CHROMATIC ABERRATION. The edge of the lens of a camera 

 represents the outer angle of a prism. White light falling upon it is 

 decomposed into its spectral components. Objects seen upon the 

 ground-glass plate have an iridescent hue. In the eye this trouble 

 is obviated by the presence of the iris and the fact of the edge of 

 the lens being more angular and less curved. 



VISUAL ANGLE. It is usually stated to be the angle included 

 by the lines from the extreme points of the object where they cross 

 at the nodal point. The apparent size of the object depends upon 

 the visual angle. Acuteness of vision is inversely as the size of the 

 visual angle. 



Act of Accommodation, When a luminous body is brought too 

 near to the eye, the rays which pass from it tend to come to a focus 



