Captain Abney on Photographic Irradiation. 



47 



When the construction of the sensitive film is taken into 

 account, the cause of irradiation (or blurring of the image) is 

 not far to seek. 



It is these small particles that cause the film to be translucent 

 and prevent its being transparent. Where there is transparency 

 there must be a scattering of the incident rays. 



These particles are quite measurable; hence we cannot treat 

 the question as if they were small fractions of a wave-length. In 

 fact they are several times larger in diameter than the greatest 

 wave-length ; and as we have to deal with the smaller wave- 

 lengths, we may treat the question as a case of simple geometrical 

 reflection. At any rate, the dispersion caused would not inter- 

 fere with the general result. 



The physical forms which the particles take are not quite ap- 

 parent. Being in an amorphous state, it will not be unfair to 

 assume that they are generally of the shape of a spheroid. For 

 the sake of calculation, we may also assume that each particle 

 reflects a definite proportion of light. This hypothesis will be 

 found not to affect the aspect of the problem, if the refracted 

 rays be traced after transmission through the sphere, in a similar 

 manner to that in which it is proposed to trace the reflected 

 rays. In any case it is only assumed that an approximate solu- 

 tion can be obtained. 



Let us consider the action of one particle, and, for the sake of 

 simplicity, let us take it to be one in contact with the surface of 

 the glass plate on which the film rests. The case where the 

 minimum irradiation will be produced is that in which the inci- 

 dent ray falls perpendicularly on the surface. First, on these hy- 

 potheses let us examine the intensity and duration of the light 

 that would fall on the plate. 



As the general direction of the light is perpendicular to the 

 surface, it is evident that by far the largest proportion of it must 

 fall in the same direction on the particle under consideration. 



Fig. 1. 





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