2l8 A. T. CAMERON AND C. H. o'DONOGHUE. 



of illumination persists much longer. Cessation of stimulus there- 

 fore appears to be more powerful than application of stimulus. 

 Ether anaesthesia abolishes the response to increase of illumination 

 first. 



' The maximum effect is reached at from twenty to twenty-five 

 minutes after injection, and the reflex disappears in another twenty 

 to sixty minutes, or occasionally after even a longer interval. 

 Quality, of light does not affect the result ; both red and blue light 

 evoke the reflex. Intensity of light appears to be the governing 

 factor. A retina fatigued by the shining of a strong light upon it 

 for some minutes does not call forth such an active response on 

 switching off the light. There may be no movement, but after a 

 minute or two the response can be again elicited by further light 

 stimulation. The degree of response varies in different animals 

 from a slight head movement to such vigorous movements of the ' 

 hind limbs as would be excited by a strong electric current applied 

 externally. After the reflex has disappeared electrical stimulation 

 still produces active muscular responses. . . . The optimum dose 

 of benzene appears to be 1.2 per cent, of the body weight. Two 

 per cent, induces convulsions and death rapidly, and the retinal 

 reflex can not be observed, while 0.7 per cent, may not produce 

 any definite effect. . . . Similar changes have been observed with 

 toluene, and to a less extent with phenol. Chlorbenzene and brom- 

 benzene give the reflex when injected in somewhat larger doses. 

 The reflex could not be detected after injection of similar doses of 

 xylene, aniline, nitrobenzene, benzyl alcohol, and pyridine." 



At the same time Frohlich and Kreidl published an account 2 of 

 similar observations on the prawn Palccmon squilla F. found under 

 stones at low tide. These observations were carried out on the 

 island of Brioni in the Adriatic, and initially were accidentally 

 made during experiments to determine the action of certain drugs 

 on the heat-narcosis of these prawns. Frohlich and Kreidl found 

 that a similar reaction to illumination changes develops in these 

 animals when immersed in sea-water containing camphor or 

 phenol. A small arc-light was used for illumination. Thus for 

 camphor : 



' Immersion in camphor sea-water solution of concentration 

 i : 16000 is fatal to Palccmon in from five to fifteen minutes. The 



