io8 



SCIENC E- G OSSIP. 



THE PHOTOGRAPHY OF COLOUR. 

 By E. Saxger Shepherd. 



{Continual from page 69.) 



T~\URING the last forty years very many 

 -'-^ attempts have been made to secure photo- 

 graphs of objects in their natural colours. Glowing- 

 accounts have been published announcing a perfect 

 solution of the problem, only to be followed within 

 a few weeks by contradictions and frequently the 

 exposure of some ingenious fraud, until the very 

 mention of a photograph in colours is looked upon 

 with suspicion. To-day, however, photography in 

 colour no longer means a photograph printed in 

 colours, nor yet an ordinary photograph coloured by 

 hand, both of which terms are apt to be associated 

 in the artistic mind with recollections of attempts 

 of a very unsatisfactory nature. 



by the second method in accuracy of colouring. 

 An additional disadvantage is that the image can 

 only be seen under particular conditions of light 

 ing and when viewed at a particular angle. 



Processes based upon the second method are 

 however, of a far more practical nature. The 

 Young-Helmholtz theory is based upon the fact 

 that all the colours of the spectrum, and therefore 

 all the colours found in nature, may be counter- 

 feited sufficiently nearly to deceive the human eye 

 by mixtures of three colours of the spectrum itself 

 — a particular red, a particular green, and a par- 

 ticular blue-violet. 



First a word about the primary colours — red, 



u mm 



■ •'&/w a 



Figs. 10, 11, 12. ( Vide p. 112.) 



As we speak of it to-day, it means the final out- 

 come and practical result of long series of scientific 

 experiments carried out by various able investi- 

 gators during the last thirty or forty years. All 

 the methods which have stood the test of time 

 may, however, be classed under two heads: — 

 (1) Those based upon Interference, as the process 

 of Professor Lippmann of Paris : and (2) those 

 based upon the Young-Helmholtz theory of tri- 

 chromatic vision. 



The first process — the Interference method of 

 Professor Lippmann— produces a single positive in 

 the camera, which is incapable of being reproduced; 

 and, as the process is intricate and the requisite 

 exposure long, few results have been shown, and 

 even these have been very inferior to photographs 



green, and blue-violet. Even at the present day 

 people find a difficulty in accepting red, green, and 

 blue-violet as the primary colours, this difficulty 

 arising from their ideas of colour mixture being 

 derived from the amalgamation of pigments upon 

 paper. 



In order to see a coloured object we have to 

 illuminate it by white light, and white light con- 

 tains light of all colours. As we have already seen 

 by means of the spectroscope, we can separate 

 white light into its component colours, and in 

 fig. 6 we have a drawing of the spectrum showing 

 curves which, by their height above the base line, 

 represent the power of the three primary colour 

 sensations to affect the human eye. 



If we take three coloured glasses, a red, a green, 



