1903.] On the Sensation of Light produced ly Radium Rays. 395 



bleaching action whatever on the visual purple of the eye of the f log 

 or rabbit. 



The method we adopted was to keep the animal in the dark for 

 some hours, to kill it and dissect out the retinae in sodium light, and 

 to expose one retina to the rays from 50 milligrammes of pure radium 

 bromide in a moist dark chamber, while the other retina remained in 

 a similar dark chamber as a control. 



The retinae, when removed, were spread on to thin sheets of mica 

 and placed over the radium at about 3 mm. distance", in such a way 

 that either the mica alone was between the radium and the retina, 

 or the mica and a layer of opaque black paper to screen off the light 

 rays. In a few cases the retina was so placed that nothing more 

 than about 2 mm. of air separated it from the radium. The time of 

 exposure was, as a rule, 20 hours. In no case was there any difference 

 in tint between the exposed and the unexposed retinas, while both 

 control and exposed retinae were found to bleach in full daylight in a 

 few seconds. 



It will be noticed that in those experiments in which the light rays 

 from the radium were allowed to fall on the retina, no bleaching 

 could be detected. This does not prove that they were wholly without 

 action since, as Kiihne showed, even the excised retina has a feeble 

 power of reproducing the purple. The experiments prove merely 

 that the a, /?, and 7 rays, either alone or with the light rays, fail to 

 overcome this recuperative power sufficiently to effect a detectable 

 decrease in the quantity of pigment in 20 hours. 



When one considers the density and activity of the stream of 

 "invisible" rays at 3 mm. distance from 50 mm. of radium bromide, we 

 may fairly conclude that these rays have no action on the pigment. 

 This conclusion led us to consider whether the sensation of light 

 which is caused by the invisible rays, is, indeed, due to the direct 

 response of the retina. When one considers how limited in extent is 

 that portion of the spectrum to which the retina responds, it would be 

 a matter of surprise if so specialised an instrument were found to 

 respond to rays as different from light-waves as are the radium rays. 

 The evidence we have been able to obtain points to the conclusion 

 that the retina probably does not respond directly to the radium rays, 

 but to light rays, which are given out by the tissues of the eyeball 

 chiefly the cornea and lens when they are traversed by the ft and 

 7 rays. 



The fresh lens of a sheep, ox, or rabbit was found to glow strongly 

 when exposed to the rays. The cornea and vitreous humour also 

 glow, but to a less extent, and the retina itself gives a strong glow. 

 The sclerotic, on the other hand, glows very slightly. The glow of the 

 lens alone is so striking as fully to account for the sensation of light 

 produced by the rays. 



