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LXXI. On the General Problem of Photographic Reproduction, 

 ivith suggestions for enhancing Gradation originally invisible. 

 By Lord Rayleigh, O.M., F.B.S* 



IN copying a subject by photography the procedure usually 

 involves two distinct steps. The h'rst yields a so-called 

 negative, from which, by the same or another process, a 

 second operation gives the desired positive. Since ordinary 

 photography affords pictures in monochrome, the repro- 

 duction can be complete only when the original is of the 

 same colour. We may suppose, for simplicity of statement, 

 that the original is itself a transparency, e. g. a lantern-slide. 



The character of the original is regarded as given by 

 specifying the transparency (t) at every point, i. e. the ratio 

 ot light transmitted to light incident. But here an ambiguity 

 should be noticed. It may be a question of the place at which 

 the transmitted light is observed. When light penetrates a 

 stained glass, or a layer of coloured liquid contained in 

 a tank, the direction of: propagation is unaltered. If the 

 incident rays are normal, so also are the rays transmitted. 

 The action of the photographic image, constituted by an 

 imperfectly aggregated deposit, differs somewhat. Rays 

 incident normally are more or less diffused after transmission. 

 The effective transparency in the half-tones of a negative 

 used for contact printing may thus be sensibly greater than 

 Avhen a camera and lens is employed. In the first case all the 

 transmitted light is effective ; in the second most of that 

 diffused through a finite anole fails to reach the lensf. In 

 defining t — the transparency at any place — account must in 

 strictness be taken of the manner in which the picture is to 

 be viewed. There is also another point to be considered. The 

 transparency may not be the same for different kinds of light. 

 We must suppose either that one kind of light only is em- 

 ployed, or else that t is the same for all the kinds that need 

 to be regarded. The actual values of t may be supposed to 

 range from 0, representing complete opacity, to 1, repre- 

 senting complete transparency. 



As the first step is the production of a negative, the 

 question naturally suggests itself whether we can define the 

 ideal character of such a negative. Attempts have not been 

 wanting ; but when we reflect that the negative is only a 

 means to an end, we recognize that no answer can be given 

 without reference to the process in which the negative is to 



* Communicated by the Author. 



t In the extreme case a negative seen against a dark background and 

 lighted obliquely from behind may even appear as a positive. 



