46 Captain Abney on Photographic Irradiation. 



form, 



- (3{wc n )~ +2 —^ 3 (wc n ) IT + . . .J , 



and express the meaning of this equation as follows : — If f szV/- 

 m/?es a function whose form is determined by the equation 



O re— 1 ™2 O n — 2 



/( ff ) = ?/S<7— + 2^^-TT+.. ; , . . (61) 



then is 



h = c~ n f(wc n ) (62) 



This latter we can forthwith extend in such a way as to deter- 

 mine the quantity V = m(h l -\- h Q +h 3 ), viz. 



U = m tc 7 »j\wtf) + c-"/K^) + e 3 -/KcJ) ] , 

 or, employing the sign of summation, 



V = m2c- n f{iuc n ) (63) 



In order to derive from this expression of U the expressions of 

 the two other quantities requiring determination, we need only 

 put 



E = tJ + T = U + mSw, ..... (64) 



U-T = U-m2w . (65) 



[To be continued.] 



VII. On Photographic Irradiation. 

 By Captain Abney, R.E., F.R.A.S., F.C.S* 



THE most frequently received notion regarding photographic 

 irradiation (i. e. an increase on development of the appa- 

 rent size of the image of a luminous object when photographed 

 against a darker body) is, that it is due to simple reflection of 

 the incident rays from the back of the plate. This view is quite 

 untenable, excepting in the case where the incident rays fall at 

 an angle with the perpendicular to the surface of the plate. If 

 a bromo-iodized film be sensitized in an ordinary nitrate-of-silver 

 bath and be examined with a microscope, using a high power, 

 it will be found that it consists of particles of bromo-iodide of 

 silver separated by considerable intervals one from the other. 

 The collodion vehicle is illuminated with that peculiar greenish- 

 yellow tint which always marks a film of this description. A 

 few simple experiments will show that this colour is entirely 

 due to the light reflected from the particles, and that it is not 

 due to the collodion itself. 



* Communicated by the Author. 



