50 



Captain Abney on Photographic Irradiation. 



sities by the method described in my paper in the Philosophical 

 Magazine for September 1874, 1 obtained two curves which, when 



Fig, 3. 



the sun's diameter was taken into account, as also the irradiation 

 due to the cause which I shall presently explain, gave a near 

 coincidence to that obtained by this theory. It should be noted 

 that the image obtained on the bottom film was that due to the 

 light refracted and not reflected. In the films employed, about 

 eV of the actinic rays were transmitted through the first plate, and 

 the amount of reflection from the sensitive film itself was nearly 

 •5- of the whole light. 



(iv) With dry plates I also made other experiments. I cut 

 small holes and slits in platinum foil. Placing these in contact 

 with the 'plate, it was exposed to direct sunlight. With the 

 former, the rings were obtained as before; with the latter, 

 parallel lines joined with semicircles. This gave sufficient 

 evidence that nothing in the lens used in (i), (ii), and (iii) could 

 cause the effects produced. 



If light be scattered from particles, it must also take place in a 

 direction parallel to the surface of the film — that is, in the film 

 itself. Since the thickness of the film is so small, we need only 

 consider the amount that would fall on a plane at right angles 

 to the surface. With the same hypotheses as before, and treating 

 it similarly, and taking B as the origin of x along BC and 

 OB = /z, we have 



x — r sin 6 + (h— r cos 6) cot 20. 

 If we make h large compared with r, 



x=hcot2d ; 

 as before we obtain , 2/ 



and the same curve obtains as in the last case, showing that the 

 greatest intensity of light is in the direction parallel to the sur- 

 face of the film. 



When h is not large compared with r, the same result holds, 



