784 WALKER. 



However, Aubaret ^ in 1900 was inclined to disregard heating effects 

 in sunblinding since he found a thermometer held in sunlight concen- 

 trated by 40 D diaphragmed lens, only registered 1° to 2° increase in 

 temperature. But Birch-Hirschfeld ^^ showed that 50° paraffin in 

 thin layers on black paper was melted in a few seconds when exposed 

 to sunlight as the retina would be in sunblinding but wdien white 

 paper was used under the same conditions several minutes were re- 

 quired to melt the paraffin, thus demonstrating the effect of black 

 retinal pigment in absorbing heat. 



The phenomenon of light adaptation and the effect of various wave 

 lengths of light on the retina has been studied by a number of observers, 

 Mann 2^^, Kuhne^^^, Pergens^^^, Nagel^^^, van Genderen-Stort ^^'', 

 and more recently by Hess^^^, Birch-Hirschfeld^^ and others. Van 

 Genderen-Stort ^^'^ in 1887 showed that pigment wandering in the 

 choroid and retina was least active in yellow light and this finding 

 called attention to the value of yellow glasses as protection for the 

 eyes. He further amplified the knowledge of the effect of light on 

 the finer structures of the retina of the dark adapted eye. When such 

 an eye is exposed to light he showed that the pigment cells just out- 

 side the rods and cones send down processes, apparently between the 

 rods, which carry with them much pigment and thus tend to isolate 

 optically the rods from each other. At the same time the cones tend 

 to escape this isolation somewhat by moving in the same direction 

 tow^ards the light. Another change found to take place was the 

 bleaching of the visual purple found in the outer part of the rods in the 

 dark adapted eye. 



The effect of ultra violet ray and light fatigue on the finer struc- 

 tures of the retina has been studied by Widmark*^^, Czerny^^, 

 DeutschmannS^ Ogneff^sa, Bach i*, Stebepsa, Kiribuchi 203, and 

 Terrien ^^*. Except for the last three, who found a very slight gang- 

 lion cell chromatolysis, their results were negative, which Birch- 

 Hirschfeld ^'^ later thought was due to insufficient light intensities 

 having been used. In snow blinding nothing more than hyperemia 

 of the retina and optic disc have been reported (Reich ^^, 1880). 



Birch-Hirschfeld^'^ in 1904 made some experiments on rabbits in 

 which he claimed to have produced definite pathological changes by 

 exposing the eyes to ultra violet light. In his first series of experiments 

 he separated out the ultra violet light from a 15 ampere carbon arc 

 light by means of a quartz prism, and obtained retinal changes only 

 in eyes from which he had removed the lenses. In his second series 

 he used the direct light from a 3 to 4.5 ampere water cooled Finsen 



