1903.] On the Sensation of Li yht produced by Radium- Rays. 397 



spending in size fairly well with the human eyeball, it was found by 

 measurement with the electroscope that the cornea, lens, and front half 

 of the vitreous humour taken together had the same screening action as- 

 6 -6 mm. of lead. That is to say, they arrested all the /3 rays and a 

 measurable fraction of the y rays. Therefore the ft rays do not 

 penetrate to the retina, except perhaps in small quantity to the most 

 anterior portion. 



The part played by the y rays is not so obvious. It is certain that 

 they can excite the excised lens to become luminous, since a lens- 

 separated from the radium by a centimetre of lead emits light enough 

 to be visible. The fluorescence is, however, exceedingly faint, much 

 less for instance than that excited in a piece of glass of about the 

 same size. On the other hand, the sensation of light evoked by those 

 rays which traverse even 4 cm. of lead is quite considerable, and no- 

 one can fail to be struck with the fact that the fluorescence of an 

 excised lens and the sensation of light produced by the y rays are very 

 dissimilar values. It is, therefore, possible that the y rays produce 

 the sensation of light in part only when they actually strike the retina. 

 Here again, however, the response need not be directly to the y rays; 

 but to light-waves started in the retina itself, for, as has been already 

 pointed out, the excised retina glows strongly when brought near 

 radium. 



The following experiment helps to prove that the y rays produce the 

 sensation of light in great part when they actually strike the retina. 

 The radium was covered with 5 mm. of lead to cut off the /? rays, am! 

 a vertical plate of lead 40 mm. deep and 2 mm. thick was moved 

 backwards and forwards over the radium so as to form a bar-like 

 screen 40 mm. thick and 2 mm. wide between it and the eye. The 

 effect produced was that of a bar of shadow moving across the glow. 



Now the y rays are not refrangible, and therefore the vertical lead 

 screen would cast a partial shadow its own breadth upon the lens and 

 retina. The effect upon the lens would be to diminish its fluorescence,, 

 but as the light-waves are given off in all directions from the lens^ 

 there would not be a definite shadow on the retina. Therefore this bar- 

 like shadow must be due to the shadow on the retina itself. 



Reference was made in passing to the remarkable stopping power of the 

 eyelid. The human eyelid seems to be capable of arresting all the /? rays. 

 With the eyelid closed the insertion of a lead screen 9 -2 mm. thick 

 makes no* difference in the intensity of the sensation of light. This, 

 however, is true only when the eyelids are tightly closed. If they are 

 stretched by raising the eyebrows, and their thickness thereby reduced, 

 the interposition of the le^d screen produces a detectable decrease. 



* Many trials with six different individuals resulted in a strong balance of opinion 

 in favour of the screen producing no diminution. Only one person thought he 

 could detect a lessening ot the glow. 



