290 1'IIYSloLOGY FOR DENTAL STUDENTS. 



problems of vision, there are many others of a physio-psycho- 

 logical nature. Such for example are the visual judgments of 

 size, distance, solidity, and color. Judgments of size and dis- 

 Itnirc are dependent on: (1) the size of the retinal image, (2) 

 the effort of accommodation necessary to obtain sharp defini- 

 tion, and (3) the amount of haze which appears to surround tin- 

 object. Judgment of solidity depends on the fact that the 

 images produced on the two retina? are not exactly from the 

 same point of view; they are like the two photographs of a 

 stereoscopic picture. The brain on receiving these two slightly 

 different pictures fuses them into one, but judges the solidity of 

 the object from the differences in the two pictures. 



Judgments of color, or color vision, forms a subject of great 

 complexity. It apparently depends on the existence in the re- 

 tina of three varieties of cones, one variety for each of the three 

 primary colors. The primary colors are red, green and violet; 

 and by mixing them on the retina in equal proportions (as by 

 rotating a disc or top on which they are painted as sectors) a 

 sensation of white results; by using other proportions, any of the 

 other colors of the spectrum may be produced. When one of 

 these primary color receptors is absent from the retina, color 

 blindness exists. Thus if the red or the green receptors are 

 absent, the patient cannot distinguish between red and given 

 lights. Such persons cannot be employed in railway or nauti'-al 

 work. 



