716 VERHOEFF AND BELL. 



assumption is that the presence of these salts in senile cataract is a 

 result not a cause. Then, too, Burge made use of intensities of expo- 

 sure and wave lengths to which the lens is never subjected during life. 

 The cornea completely screens it from practically all the short waves 

 found effective by him. The longest waves with which he could 

 coagulate proteins were 302 [xfj. in length and the effect produced by 

 these was insignificant. 



Burge suggested also that his results might apply to glassblowers 

 cataract, overlooking the fact that the latter typically begins at the 

 posterior pole of the lens, whereas in his experiments the part of the 

 lens away from the light was little if at all affected. It is of course 

 obvious that the slight loss of transparency he sometimes observed 

 in this part of the lens could not have been due to the direct action 

 of the light, since the effective rays could not have penetrated so far. 

 The fatal objection to Burge's theory as applied to senile cataract is 

 that the ultra violet solar rays cannot reach that portion of the lens 

 where cataract generally begins, and that portion of the lens where 

 ultraviolet light has the best chance of action is affected only at a 

 late stage of development. 



Concentration of Energy in Images. 



We have already shown that superficial action of radiant energy 

 on the eye depends on the actual energy in ergs per square cm., or 

 other convenient measure, which falls upon the surface. Such value 

 is directly as the energy of the source and inversely as the square of 

 its distance. The density of incidence of energy at points within the 

 eye is obviously dependent on the amount to which the superficial 

 energy is concentrated by the refracting media, and at the retina the 

 concentration of energy is determined by the size of the image and the 

 aperture of the refracting system, which is determined by the area 

 of the pupil. In dealing with an extended source the image is corre- 

 spondingly extended and the surface density of energy in the image 

 is correspondingly reduced. Hence it is that with sources like the 

 tube of the quartz arc the image density is relatively small, while 

 with point sources or those of very small area, like the electric arc, 

 the retinal area is correspondingly small and of the total energy reach- 

 ing the pupil there is a greater concentration in the image. Within 

 limits the intensity of the effect on the retina is then directly pro- 

 portional to the intrinsic brilliancy, or radiation per unit area of the 



