350 Mr. J. H. J. Poole on the 



sometimes the reverse effect was observed, when a suffi- 

 ciently long exposure to slightly fog the plate was tried. 

 This slight fogging of the plate with long exposures was 

 probably due to a very small amount of actinic light 

 getting through the screens, and the retarding action of 

 the pigment observed may be caused by its slightly greasy 

 nature impeding development. Other attempts were aho 

 made by treating the pigment with various solvents, such 

 as water, alcohol, chloroform, etc., and treating a plate 

 with the resultant fine suspension of the black pigment, 

 as it did not appear to be soluble in any of the solvents 

 used. No result w;is obtained. Another method adopted 

 was to reduce the pigment to a very fine powder hy de- 

 hydration and subsequent grinding in an <»gate mortar, 

 the resulting powder being then Justed on to the plaie ; 

 but again no sensitising effect v\as shown. The difficulty 

 of obtaining good enough contact between the pigment 

 and the sensitive film of the plate would probably be 

 sufficient to account for the negative result in all these 

 cases, as it has been found that for ordinary sensitising li'yes- 

 to be effective, the dye must actually dye the silver-halide 

 grain, and not merely stain the plate. 



It is more difficult to deal with the visual purple or 

 rhodopsin in the retina, both on account of the much 

 smaller quantity available, and also because the rhodopsin 

 is only present in dark-adapted eyes. For this reason 

 the animal from which the eye is to be obtained must 

 be kept in the dark for some time before being killed, 

 to allow the rhodopsin to form in the retina, and all sub- 

 sequent dissections have to be carried out in the dark or 

 in a very subdued red light, as the rhodopsin quickly fades 

 when exposed to light. Professor Pringle was, however,, 

 kind enough to supply us with a solution of rhodopsin 

 in bile salts which had been prepared from dark-adapted 

 frogs' eyes. This solution was tested on the plate in the 

 usual manner. Again no sensitising action was detected : 

 in fact, with very long exposures which slightly fogged 

 the plate, a small retarding effect was noticed. This effect 

 was found to be due to the bile salts, — a solution of them 

 with no rhodopsin having the same slightly retarding effect 

 on the plate. It is unfortunate that bile s;ilts have this 

 action on the plate, as it would completely mask any slight 

 sensitising effect which the very small amount of rhodopsin 

 present might produce, and it is apparently not easy to 

 extract the rhodopsin from the retina by other means. 



These sensitising experiments thus lead to no diiect 



