862 TIME-RELATIONS OF RETINAL STIMULATION. 



the fact that its peripheral zone corresponds to the intermediate and peripheral 

 zones of the normal eye. In the violet-blind, on the contrary, the normal periph- 

 eral zone is absent. Incomplete color-blindness of these two types is character- 

 ized by a uniformly contracted central field. 



In the presence of hyperesthesia of the optic nerve resulting from cerebral 

 conditions, there is, curiously, a widening of the normal color-limits toward the 

 periphery. 



When colored objects are exceedingly small, and illuminated for only a short 

 time, the normal eye first fails to perceive red. It appears, therefore, that a 

 comparatively strong stimulus is required for the perception of red. The obser- 

 vation of Briicke's, that rapidly intermittent white light appears green, is also 

 in favor of this view, because the short duration of the stimulus is not capable 

 of exciting the red-perceiving elements of the retina. 



To Holmgren belongs the credit of having shown the necessity for examining 

 all railroad-officials and all pilots as to the trustworthiness of their color-sense, 

 as the correct recognition of red and green signal-lights is impossible for a color- 

 blind person. 



Method of Examination. Holmgren, in conjunction with Seebeck, chooses 

 embroidery-wool as the simplest material, in skeins of at least five shades each 

 of red, orange, yellow, greenish-yellow, green, greenish-blue, blue, violet, purple, 

 rose, brown, gray; it is best to have several different shades of the various 

 colors. In the examination , one strand of this yarn (for example light green or rose) 

 should be selected and put to one side. That color is chosen for which the indi- 

 vidual is to be especially tested ; and he is then requested to pick out the strands 

 whose colors resemble most closely that of the sample, and place them by its 

 side. According to the way in which he performs this task a judgment is 

 reached as to his color-sense. A more accurate determination is made by 

 means of the spectrum. 



Mace" and Nacati have measured the visual acuity for a small object when 

 illuminated by different portions of the spectrum. They compared with the 

 results of their investigation the observations on red-blind and green-blind 

 individuals. It was shown that red-blind persons find green light much brighter 

 than do normal persons. In the green-blind there is an excessive sensitiveness 

 to red and violet. It seems, therefore, that what color-blind individuals lack in 

 perception-power for one color, they gain for other colors. They possess also a 

 greater power of distinguishing variations in brightness. 



TIME-RELATIONS OF RETINAL STIMULATION. 



POSITIVE AND NEGATIVE AFTER-IMAGES. IRRADIATION. CON- 

 TRAST. 



As with irritation of every other nervous apparatus, a definite, 

 though short time elapses after the entrance of the rays into the eye, 

 before the visual effect is manifest, whether in the form of conscious 

 perception, or a reflex effect on the iris. The intensity of the impres- 

 sion, here also, will depend essentially upon the irritability of the retina' 

 and the other nervous structures. If the visual impression continues for 

 some time with the same intensity, the stimulation, after having reached 

 its culminating point, soon diminishes, at first rapidly, then more slowly. 

 If the light-stimulation of the retina is suddenly removed after it has 

 continued for some time, the retina remains for a time in an excited 

 condition, which is the more intense and persistent the stronger and 

 longer the light-stimulation, and the more sensitive the retina. After 

 every visual perception, therefore, especially when it has been quite 

 bright and sharp, a so-called after-image persists. There is recognized, 

 in the first place, the positive after-image, which persists with similar 

 brilliancy and color. 



A light-stimulus of short duration excites first a sensation of light, which 

 lasts longer than the stimulus; thereupon a negative after-image appears (in 



