658 THE SENSES 



its acuteness and localization in different areas toward the periphery. Con- 

 sidering the minimal distance apart which two luminous points must be to 

 be distinguished as two, it is found that when the image falls on the fovea 

 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 /*. The diameter of the cones is about 2 /*, so that the stimuli in 

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

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

 jacent cones. 



The area of the fovea centralis is small, from 0.5 to 1.5 mm. Outside of 

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

 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 distance 

 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 supposed 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 pigmenta- 

 tion 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 due 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 



