STIMULATION OF THE RETINA. 



779 



In poisoning with santonin, violet-blindness (yellow vision) occurs in consequence of the 

 paralysis of the violet perceptive retinal elements, which not unfrequently is preceded by stimu- 

 lation of these elements, resulting in violet vision, i.e., objects seem to be coloured violet 

 (Hufner). Such is the explanation of this phenomenon given by Holmgren. Max Schultze, 

 however, referred the yellow vision, i.e., seeing objects yellow, to an increase of the yellow 

 pigment in the macula lutea. 



When coloured objects are very small, and illuminated only for a short time, the normal eye 

 first fails to perceive red (Aubert) ; hence, it appears that a stronger stimulus is required to 

 excite the sensation of red. Briicke found that very rapidly intermittent white light is per- 

 ceived as green, because the short duration of the stimulation fails to excite the elements of 

 the retina connected with the sensation of red. 



[The practical importance of colour-blindness was pointed out by George Wilson, and again 

 more recently by Holmgren.] No person should be employed in the marine or railway service 

 until he has been properly certified as able to distinguish red from green. 



Methods of Testing Colour-blindness. Following Seebeck, Holmgren used small skeins of 

 coloured wools as the simplest material, in red, orange, yellow, greenish-yellow, green, greenish- 

 blue, blue, violet, purple, rose, brown, grey. There are five finely graduated shades of each of 

 the above colours. When testing a person, select only one skein e.g., a bright red or rose 

 from the mass of coloured wools placed in front of him, and place it aside, asking him to seek 

 out those skeins which he supposes are nearest to it in colour. 



Mace and Nacati have measured the acuteness of vision by illuminating a small object with 

 different parts of the spectrum. They compared the observations on red and green-blind 

 persons with their own results, and found that a red-blind person perceives green light as much 

 brighter than it appears to a normal person. The green-blind had an excessive sensibility for 

 red and violet. It appears that what the colour-blind lose in perceptive power for one colour 

 they gain for another. They have also a keen sense for variations in brightness. 



398. STIMULATION OF THE RETINA. As with every other nervous 

 apparatus, a certain but determinable time elapses after the rays of light fall upon 

 the eye before the action of the light takes place, whether the light acts so as to 

 produce a conscious impression, or produces merely a reflex effect upon the pupil. 

 The strength of the impression produced depends partly and chiefly upon the 

 excitability of the retina and the other nervous structures. If the light acts for a 

 long time with equal intensity, the excitation, after having reached its culminating 

 point, rapidly diminishes again, at first more 

 rapidly, and afterwards more and more slowly. 



[When the retina is stimulated by light, 

 there is (1) an effect on the rhodopsin (p. 740). 

 (2) The electro-motive force is diminished 

 ( 332). (3) The processes of the hexagonal 

 pigment-cells of the retina dipping between 

 the rods and cones are affected; thus they are 

 retracted in darkness, and protruded in the 

 light (fig. 563). (4) Engelmann has shown 

 that the length and shape of the cones vary 

 with the action of light. The cones are re- 

 tracted in darkness and protruded under the 

 influence of light (fig. 563). This alteration 

 in the shape of the cones takes place even if 



1. 



Fig. 



the light acts on the skin, and not on the eye- The con es of the retina and pigment-cells 

 ball at all.] (of the frog) as affected by light and 



After-images. If the light acts on the darkness : 1. after two days in dark- 

 eye for some time so as to excite the retina, ness ' 2 - after ten mi t in daylight. 

 and if it be suddenly withheld, the retina still remains for some time in an excited 

 condition, which is more intense and lasts longer, the stronger and the longer the 

 light may have been applied, and the more excitable the condition of the retina. 

 Thus, after every visual perception, especially if it is very distinct and bright, 

 there remains a so-called "after-image." We distinguish a "positive after- 

 image," which is an image of similar brightness, and a similar colour. 



