THE EYE AS A SENSORY APPARATUS 243 



"yellow" length may cause in us the sensation yellow, which is 

 only one more instance of the general fact that our sensations, 

 as such, give us no direct information as to the nature of external 

 forces; they are but signs which we have to interpret. 



Function of the Cones. These structures, since they are the 

 only sensitive elements of the fovea centralis, must be the recep- 

 tors for all ordinary conscious vision. Their special function is 

 doubtless the perception of color. This perception is a part of 

 all our conscious visual sensations. We never think of a luminous 

 object as being merely light; but always as having some color. 



Distribution of Color Sense over the Retina. By means of an 

 apparatus called the perimeter it is possible to determine the 

 boundaries of visual sensation in the retina. In using this ap- 

 paratus the subject with head supported in one position looks 

 fixedly at a point straight in front; the observer then brings 

 small squares of paper from the side toward the front and the sub- 

 ject reports the instant the square of paper comes into his field 

 of vision. The angle is marked on a specially prepared chart and 

 the observation repeated along different radii. By this means 

 the field of vision is mapped out. The visual field for any par- 

 ticular color can be determined similarly, the subject in this case 

 being required to report as soon as he is certain what the color 

 of the square of paper is. Such studies have brought out the in- 

 teresting fact that ability to perceive the different colors is un- 

 equally distributed over the retina. The margins of the visual 

 field are sensitive only to white and black, and to their mixtures 

 of gray; the fields for blue and yellow cover the whole area except 

 the margins ; the fields for red and green sensation are the smallest 

 of all, occupying only the central part of the field and covering 

 about half its entire surface. According to most determinations 

 the boundaries of the yellow and blue fields do not coincide ex- 

 actly, nor those of green and red; but it is quite probable that 

 they do coincide exactly in reality, and that experimental errors 

 account for their apparent divergences. 



It is clear from these observations that the cones in the central 

 part of the visual field are sensitive to all colors; that those further 

 out are sensitive to all except red and green ; and that the marginal 

 ones are insensitive to color as such, and distinguish only degrees 

 of light and darkness. 



