394 Mr. W. B. Hardy and Dr. H. K. Anderson. [Sept. 20, 



a response to the rays on the part of the optic nerve, the optic tract, 

 or the brain itself. We were led to this conclusion in the following 

 way. When the crystals of radium bromide are spread out so as to 

 form a layer about 1 mm. deep, covering a circle of about 5 mm. 

 diameter, the glow is very intense when the flat face of the layer is 

 presented to the eye,* and practically vanishes when the layer is turned 

 edgeways. This must be due to the fact that the volume of the stream 

 of rays is roughly proportional to the surface of the mass of the salt, 

 and that the stream is densest in a plane normal to the surface. The 

 effect of rapidly rotating the little plate of crystals before the eye is 

 very striking ; it is as though a series of blinding flashes of light were 

 thrown on to the retina. 



If now the radium be moved about over the surface of the head, 

 one notices that for the sensation of light to be present the axis of 

 greatest ray density must cut one or other eyeball, and that the 

 further the radium is removed from an eyeball the weaker is the 

 sensation. 



This statement can be illustrated by the following case. When the 

 radium is held with the face of the crystals opposite the middle of 

 the forehead, there is at most a luminosity so feeble as to be detected 

 with difficulty ; at the same time no other sensations are evoked. 

 If, however, the plate of crystals be rotated downwards and outwards 

 so as to face towards one eyeball, a strong light is at once seen with 

 that eye. 



Experimenting in this way, we convinced ourselves that no sensa- 

 tions are directly produced by the rays reaching the brain substance,! 

 and that the origin of the sensation of light lies solely in the eyes 

 themselves. 



On passing directly from daylight into a dark room one's eyes are 

 at first almost completely insensitive to the rays, and sensitiveness 

 grows only slowly. In the evening, after exposure for some hours 

 to yellow artificial light, the sensation of light is felt almost im- 

 mediately the ordinary sources of light are extinguished. Kadium 

 vision, if we may so call it, will continue to increase for a full hour 

 in the dark in other words, it is a phenomenon of the dark-adapted 

 eye. 



liadium vision, therefore, resembles the response to light of low 

 intensity, and as a great many facts go to show that this is connected 

 with the visual purple, we examined the eft'ect of the rays upon 

 that substance. To our surprise we were unable to detect any 



* To avoid repetition, it must be understood that the crystals are always covered 

 with opaque paper or cardboard, so as to exclude the actual light rays. 



t This statement is of course limited to the stream of rays from 50 milligrammes 

 of radium bromide. It is possible that a more powerful stream might evoke direct 

 response from the brain. 



