1903.] On the Sensation of Light produced T)y Radium Hays. 393 



shows that the constant term in the curvature is, in general, increased 

 numerically by the correction. Further, this constant term involves 

 the two elastic constants. 



The general conclusions drawn with regard to the beam under a 

 uniform load are in no way affected. 



" On the Sensation of Light produced by Radium Rays and its 

 Relation to the Visual Purple." By W. B. HAKDY, F.R.S., 

 Caius College, Cambridge, and H. K. ANDERSON, M.D., Caius 

 College, Cambridge. Received September 20, 1903. 



It is now well known that when a few milligrammes of a salt of 

 radium are brought near the head in the dark a sensation of diffuse 

 light is produced. We have examined this phenomenon with the 

 object (1) of determining the place of origin of the sensation; (2) of 

 identifying the particular rays which cause it. 



The rays from radium falling upon the skin produce no sensation, 

 and we failed in evoking sensations of sound, smell, or taste by their 

 agency. The only response immediately traceable to them seems to 

 be this one of diffuse luminosity. 



It may be described as an appearance of diffuse light of steady 

 intensity disposed in the external space in front of the head and filling 

 that space fairly or quite uniformly. If the radium, covered of course 

 with some opaque screen such as, for instance, black cardboard, to cut 

 off the pale light which it emits, be held in front of one eye, one 

 notices that the intensity of the glow is considerably reduced by 

 closing the eyelid. When the eye is open it is possible in a very 

 general way to locate the radium from the fact that the sensation is 

 strongest when the axis of vision is directed to it and diminishes when 

 the head is turned to one side. The sense of direction arises solely 

 from variations in the intensity of the glow and not from variations 

 in its quality. 



When the eye is closed the sense of locality is completely lost. This 

 is due to the fact, to be dealt with more fully presently, that the 

 glow is due to the /3 and 7 rays, and that the eyelid is peculiarly 

 opaque to the former, stopping apparently the whole of them. The 

 7 rays, on the other hand, having a very great penetrating power, 

 pass almost equally well through the eyelid or through the bones and 

 other tissues forming the orbit. There is, therefore, no possibility of 

 a differential screening action when once the eyelids are closed and 

 the /3 rays stopped, with the result that all sense of the direction of 

 the source of light is lost. 



The sensation of light is purely of retinal origin. It is not due to 



