EFFECTS OF RADIANT ENERGY OX THE EYE. 717 



source. The mere lowering of the intrinsic radiation by spreading 

 out the source therefore greatly lessens any possible effects of energy 

 which may reach the retina, while it does not in any way affect the 

 radiation which may reach the cornea and conjunctiva. There is, 

 however, a very notable limitation to this principle which comes 

 into play in considering small and intense radiants, as was long ago 

 shown by Charpentier ^^. ^Yhen the image of a luminous object 

 reaches the diameter of approximately 0.15 mm., variations of intrin- 

 sic radiation at the source cease to be significant and the appar- 

 ent intensity of the source varies simply with the inverse square of its 

 distance. This corresponds to the visual angle of about 40 minutes 

 of arc. For areas of greater dimensions one must reckon with the 

 size of the image as determined by the ordinary laws of geometric 

 optics, but for radiants of less than this dimension the image may be 

 taken as of constant area corresponding to the circle of diffusion, and 

 the energy concentrated in it varies as the inverse square of the 

 distance of the radiating source. In any case the energy reaching the 

 retina is diminished by the absorption in the media of the eye of which 

 we will now take account. 



General Nature of Absorption. 



By absorption one means in general terms the stoppage of energy 

 in any medium. This may be either specific, affecting only energy of 

 certain ware length, or general, affecting more or less all energy what- 

 ever. In the former case it is due to the molecular or atomic struc- 

 ture of the material, in the latter to the fact that it is not physically 

 homogeneous. In specific absorption such as takes place, for 

 instance, in colored glasses, the molecular structure is such as to 

 respond to and take up certain particular oscillation frequencies so 

 that waves of these frequencies do not readily pass through the sub- 

 stance. In general absorption the substance contains particles which 

 reflect the energy from their surface or absorb it without definite 

 regard to its wave length. Such absorption occurs, for instance, 

 in some glass which is full of microscopic bubbles which reflect the 

 energy at their surfaces, or in certain dark glasses which are filled with 

 minute opaque particles. Both kinds of absorption may coexist in 

 the same material, but. general absorption involves it only in a very 

 indirect way due to the general properties of reflecting surfaces. 



The stoppage of radiant energ\- in the media of the eye is of two 



