SIGHT 235 



It is easily demonstrated for man that luminosity is recog- 

 nized more easily than color, and this probably holds true for 

 all organisms. Colored objects appear colorless when the light 

 is too feeble, and if the light is increased in intensity the colors 

 appear, but as it becomes too strong there is a tendency for all 

 colors to pass into white. This is most noticeable in the yellow. 

 Different regions of the retina vary also in their power to dis- 

 tinguish colors. Red is lost at a short distance from the macula 

 lutea, while the violet is lost only at the borders of the retina. 



Stimulation of the retina is followed normally by a latent 

 period, then by a period during which the effect of the excita- 

 tion reaches a maximum; from the maximum there is a slow 

 decline in the effect, which is analogous to fatigue, and when 

 the stimulation has ceased there is an after-effect which slowly 

 passes away. When a very bright object is looked at for some 

 time, the impression upon the retina lasts for a considerable 

 interval after the excitation has ceased. This is called the posi- 

 tive after-image. Both form and color are visible. The latter 

 generally passes through a series of colors green, blue, violet, 

 purple, and red before it disappears. The negative after-image 

 depends upon fatigue of the retina, and differs from the positive 

 after-image in that its color is the complementary of that of 

 the object. The effect produced by an object upon the retina 

 depends in part upon the amount it has been fatigued by pre- 

 vious impressions. This makes the field of vision appear darker 

 near a light area and lighter near a dark area than it really is, 

 while color is so modified by the neighboring field that it 

 appears the complementary of the latter. 



Ordinary white light, if decomposed, is resolved into the 

 seven primary colors violet, indigo, blue, green, yellow, orange, 

 and red. Each of these primary colors has a different wave 

 length. Colors other than the seven primary colors are the 

 result of the mixture of two or more of the primary colors in 

 various proportions. Nothing is known of the manner in which 

 the rods and cones are made to vibrate by ordinary images, 

 nor as to the nature of the process by which the different 

 color effects are conveyed by the optic nerve. The different 

 color theories assume that there are different substances in the 

 retina capable of responding to different wave lengths of light 

 (comparable to a photochemical process). Red, green, and 



