1882.] On the Cause of the Light Border in Photographs, tyc. 67 



more or less on the particular substance acted on; but taking the 

 preparations most usually employed, it may be said to lie about the 

 indigo or violet. Such an objective would be much under-corrected 

 for the red, which accordingly would be much out of focus, and the 

 ultra-red still more so. 



When such a camera is directed to a uniform bright object, such as 

 a portion of overcast sky, the proportion of the rays of different 

 refrangibilities to one another is just the same as if all the colours 

 were in focus together ; but it is otherwise near the edge of a dark 

 object on a light ground. As regards the rays in focus, there is a 

 sharp transition from light to dark ; but as regards rays out of focus, 

 the transition from light to dark though rapid is continuous. It is, 

 of course, more nearly abrupt the more nearly the rays are in focus. 

 Just at the outline of the object there would be half illumination as 

 regards the rays out of focus. On receding from the outline on the 

 bright side, the illumination would go on increasing, until on getting 

 to a distance equal to the radius of the circle of diffusion (from being 

 out of focus) of the particular colour under consideration the full 

 intensity would be reached. Suppose, now, that on the sensitive 

 plate the rays of low refrangibility tend to oppose the action of those 

 of high refrangibility, or say act negatively, then just outside the 

 outline the active rays, being sharply in focus, are in full force, but 

 the negative rays have not yet acquired their full intensity. At an 

 equal distance from the outline on the dark side the positive rays are 

 absent, and the negative rays have nothing to oppose, and therefore 

 simply do nothing. 



9. I am well aware that this explanation has need of being con- 

 fronted with experiment. But not being myself used to photographic 

 manipulation, I was unwilling to spend time in attempting to do what 

 could so much better be done by others. I will, therefore, merely in- 

 dicate briefly what the theory would lead us to expect. 



We might expect, therefore, that the formation of the fringe of 

 extra brightness would depend : — 



(1.) Very materially upon the chemical preparation employed. 

 Those which most strongly exhibit the negative effect on exposure to 

 a spectrum after a brief exposure to diffuse light might be expected to 

 show it the most strongly. 



(2.) Upon the character of the light. If the light of the bright 

 ground be somewhat yellowish, indicating a deficiency in the more 

 refrangible rays, the antagonistic effect would seem likely to be more 

 strongly developed, and, therefore, the phenomenon might be expected 

 to be more pronounced. 



(3.) To a certain extent on the correction of the objective of the 

 camera. An objective which was strictly chemically corrected might 

 be expected to show the effect better than one in which the chemical 



p 2 



