VISUAL PURPLE 745 



the two points may be very near together, as little as one minute or even less. 

 Two stars can be seen only at a somewhat greater angular distance, two to- 

 three minutes. One minute angular measure covers an area on the retina of 

 a trifle over 4/4. The diameter of the cones is about 2/y., so that the stimuli in 

 the fovea fall on at least two separate cones. The inference seems reason- 

 able that the retina in its most sensitive part can localize stimuli that fall on 

 adjacent cones. 



The area of the fovea centralis is small, from o . 5 to i . 5 mm. Outside of 

 its area the acuteness of vision quickly falls off. The fact is roughly esti- 

 mated by fixing the vision on a letter in the printed line in the book before the 

 reader and then determining the number of letters to either side that can be 

 identified. The height of these letters is 1.5 mm.; by measuring the dis- 

 tance of the page from the eye one can quickly calculate the area of distinct 

 vision on the retina. Test types are printed on the basis of an angle of five 

 minutes. 



In the outer limits of the retina the power of localizing stimuli is very 

 slight; in fact, in the extreme borders of the field it is difficult to determine 

 other than general form. 



Visual Purple. The method by which a ray of light is able to stimulate 

 the endings of the optic nerve in the retina is not yet understood. It is sup- 

 posed that the change effected by the agency of the light which falls upon the 

 retina is in fact a chemical alteration in the protoplasm, and that this change 

 initiates a nerve impulse that is transferred to the optic nerve endings. The 

 discovery of a certain temporary reddish-purple pigmentation of the outer 

 limbs of the retinal rods in certain animals, e.g., frogs, which had been 

 killed in the dark, forming the so-called rhodopsin or visual purple, appeared 

 likely to offer some explanation of the matter, especially as it was also found 

 that the pigmentation disappeared when the retina was exposed to light and 

 reappeared when the light was removed, and that it underwent distinct 

 changes of color when other than white light was used. It was also found 

 that if the operation were performed quickly enough and in the dark, the 

 image of an object, optogram, might be fixed in the pigment on the retina by 

 soaking the retina of an animal in alum solution. 



The visual purple cannot, however, be absolutely essential to the pro- 

 duction of visual sensations, as it is absent from the retinal cones, and from 

 the macula lutea and fovea centralis of the human retina, and does not appear 

 to exist at all in the retinae of some animals, e.g., bat, dove, and hen, which 

 are, nevertheless, possessed of good vision. 



However, the fact remains that light falling upon the retina bleaches the 

 visual purple, and this must be considered as one of its effects. It has been 

 found that certain pigments, also sensitive to light, are contained in the inner 

 segments of the cones. These colored bodies are said to be oil globules of 

 various colors red, green, and yellow called chromophanes, and are found 



