VISION. 



793 



The luminous impression consists of the vibrations of the lumi- 

 nous ether which stimulate the outer portion of the rods and cones. 

 In them there is produced a molecular, mechanical change, or dis- 

 turbance. Whenever the layer of rods and cones is stimulated, the 

 excitation is propagated from without inward to all of the retinal 

 elements. 



Von Kries holds that the cones alone have the power to per- 

 ceive colors (day vision), whilst the rods are sensitive only to light 

 and darkness. The rods, by their adaptability in the dark through 

 the regeneration of their visual purple, form the special apparatus 

 for vision in dim lights (night vision). The various elements are 

 connected by fibers, and, finally, by the optic nerve with the brain. 



Physiology of the Eye. 



The study of the phenomena of the eye may be divided into 

 four parts: (1) dioptrics, (2) accommodation, (3) imperfections and 

 corrections, and (4) vision with both eyes. 



Dioptrics. The eye has previously been mentioned as being like 

 a camera obscura. If a small opening exist in the shutter of a dark 

 room the rays of light from the outside passing through the opening 

 will form an inverted image of the external object upon the opposite 

 wall of the chamber. However, unless the opening be very small, 

 the image will be blurred and indistinct. These latter. qualities will 

 be due to overlapping of rays of light from various points of the 

 object. If the opening be small enough the overlapping rays will 

 be cut off and a distinct image be formed. Should a convex lens be 

 interposed in the path of the rays of light the opening may be very 

 considerably enlarged, and yet the various rays be brought to a focus 

 so that diffused images will be prevented. 



The camera obscura is popularly known to-day in the form of 

 the photographic camera. The latter consists of a box blackened on 

 the interior to prevent reflection from the walls. In front is a short 

 tube which contains achromatic lenses. In the back wall of the 

 camera is found a ground-glass plate upon which the image formed 

 by the lens is focused. If the camera be so adapted that parallel 

 rays falling upon the lens are focused upon the ground-glass plate, 

 then divergent rays must have their focal point behind the plate. 

 Should the plate be moved backward or forward the focal point can 

 be made to coincide with the conjugate focus of the rays diverging 

 from the object. 



