Captain Abney on Photographic Irradiation. 49 



beyond the visual angle will do so. Calculating according to 

 Fresnel's formulae the amount of light that would be reflected 



for each angle less than the critical angle, we get the curve which 

 bouuds the shaded part of the figure. The area between the 

 two curves shows the amount that would pass through. The 

 refractive index has been taken as 1*5. 



An examination of the figure shows that if a ray of light fall 

 on the film, on development a ring might be formed round 

 the image at a distance from it equal to 2t cot^, where % is the 

 critical angle ; on the inside it would be shaded off more abruptly 

 than on the outside. If then a cylindrical pencil of rays strike 

 a film perpendicular to its surface, a similar detached annulus 

 would result, provided the diameter were < 2t cot %. 



To ascertain if actual results held with the theory, the fol- 

 lowing experiments were made : — 



(i) A short-focus lens (about 6 inches) was employed to form 

 an image of the sun on a sensitive plate whose thickness was 

 approximately '12 inch. On developing the image a well-de- 

 fined annulus was obtained, the most intense portion being equal 

 to the sun's diameter. The shading-offwas that described above, 

 and answered to the theory. 



(ii) A plate of double the thickness of the foregoing was 

 similarly treated. The mean diameter of this annulus was 

 double that of the last. 



(iii) Two plates, similar to that used in (i), were next experi- 

 mented with. Each one carried a sensitive film. The film of 

 the bottom plate, 1 (fig. 3), was separated from the surface of the 

 top plate, 2, by a strip of thin card to prevent contact. On de- 

 veloping them the ring was obtained on the first plate, and a 

 diffused image on the second. 



Throwing the two images on a screen, and taking the inten- 

 PhiL Mag. S. 4. Vol. 50. No. 328. July 1875. E 



