True Photographic Rendering of Contrasts. 151 



Thus both laws of refraction are satisfied on the above 

 assumption. 



In the limiting case when the absolute index of refraction 



Av 

 of the first medium is unity, the expression c : -„ becomes 



/V 

 c — Av, where c is the velocity of light in the aether, and v 

 the velocity of the second medium. But the relative velocity 

 must then be c—v. Thus A = l, and hence the velocity 

 of light relative to any medium moving with velocity 





— o I , whe 



re 



v is c 1 -=, the actual velocity being a + v 1 1 -5 | 



/*i \ /*i / 



Ci is the velocity of light in the medium when v = 0, and/Zj is 

 the absolute index of refraction of the medium. The case 

 of reflexion, being a particular case of refraction, is included 

 in the above discussion. Refraction in crystalline media 

 can be similarly treated. 



XII. The Fundamental Lara for the true Photographic 

 Rendering of Contrasts. By F. F. RenwicK, A.C.G.I., 

 F.I.CS " 



THE paper by Prof. Porter and Dr. Slade bearing the 

 above title, in the July number of the Philosophical 

 Magazine (vol. xxxviii. p. 187, 1919), on a method of 

 deriving the relations which must hold good between the 

 "characteristic" curves of the negative and positive materials 

 if exact reproduction of the tones of the original subject is to 

 be obtained, has shown me that the more general method which 

 I adopted in treating this part of my subject in the nine- 

 teenth Traill Taylor Memorial Lecture (Phot. Journal, vol. lvi. 

 p. 222, 1916) is not self-evident, as I had supposed. 



Inasmuch as my method, unlike theirs, is not restricted to 

 exact nor even to proportionally correct reproduction, but 

 can be employed with equal facility for any desired relation 

 between the original and the final copy, it is probably worthy 

 of record. 



The relations between the incident light and the effects 

 produced in the subsequent development process are usually 

 represented by Hurter and Driffield's method of "cha- 

 racteristic curves/' in which the abscissae are values of 

 logE (E, the exposure, being the product of intensity I and 

 time t), while the opacity-logarithms to base 10 (generally 



* Communicated bv the Author. 



