396 Mr. AY. B. Hardy and Dr. II. K. Anderson. [Sept. 20, 



This quality of emitting light under the influence of the rays is not 

 at all peculiar to the tissues of the eyeball. The skin glows, as may 

 readily be seen by bringing the hand near to radium covered by 

 opaque paper. Fat and muscle glow strongly. 



The fact that direct stimulation of the retina contributes at most but 

 little to the sensation of light, comes out clearly when the nature of 

 the rays which cause the sensation and their power of penetrating the 

 eyeball are examined. From this investigation the a rays were always 

 excluded. They possess so little penetrating power that they are 

 arrested by the thickness of opaque paper which is necessary in order 

 to screen off the light rays. 



The ft and y rays, therefore, remain for consideration, and they 

 both contribute to the production of the sensation of light. 



The following experiment proves this : 



Fifty milligrammes of radium bromide, covered with opaque paper, 

 were placed at the bottom of a thick- walled lead tube 6 cm. in length, 

 on each side of which were the poles of an electric magnet giving a 

 field of more than 5000 lines over nearly the entire length of the lead 

 tube. The physical effect of this field would be to divert the ft rays into 

 the lead. The optical effect was very striking. When the field was 

 -established while the eye was applied to the tube, the glow suddenly 

 diminished to what appeared to be about a-fif th or less of its previous 

 intensity. The change resembles the sudden turning down of a gas 

 light. The persistent or residual glow must be that due to the 

 y rays. 



The effect of the y rays can be demonstrated by screening off the 

 ft rays with lead screens. A plate of lead 2 -3 mm. thick was found to 

 reduce the intensity of the glow very much. Such a plate cuts off 

 the fi rays. The further addition of four more such screens, thus 

 increasing the thickness of the lead to 11 '5 mm., scarcely diminishes 

 the remaining glow, for they arrest only a small fraction of the y rays. 

 In these screen experiments it is, of course, necessary to keep the 

 distance between the radium and the eye constant. This was effected 

 by a simple apparatus designed for the purpose which does not need 

 description. 



The glow is still quite strong when 4 cm. of lead are interposed 

 between the eye and the radium (50 milligrammes), but it is faint 

 with more than 5 em. 



Similarly the fluorescence of the excised lens of the eye which is 

 excited by those rays which traverse lead 2*3 mm. in thickness is not 

 sensibly diminished by adding two more such screens, thereby increasing 

 the thickness to 6 '9 mm. 



So far as the ft rays are concerned, it is certain that they cause a 

 sensation of light solely by inducing fluorescence of the tissues of the 

 eyeball in front of the retina. Taking the eyeball of the sheep as corre- 



