CHAP, vi] PHOTOGRAPHY. 295 



a powerful lamp ; gradually the image more grew distinct, and after 

 one or two minutes came out very strongly. The thin coating of gold 

 which covered the proof, by strengthening the tones, protests J the 

 picture from accidental changes. Daguerre's process required, as we 

 have seen, a rather long exposure, on an average a quarter of an hour 

 - to the sun's rays. Attention was naturally directed to reducing this 

 time, which on many accounts, limited the employment of the method. 

 For portraits of living people and animals, or for moving objects, it was 

 very important to solve this problem, which was in fact to discover 

 compounds more rapidly impressible than iodide of silver. Several 

 were found, and they were called accelerating substances, because they 

 aided the action of the iodine. 



In 1840 Goddard, and in 1841 Claudet, found that the iodized plate, 

 exposed to the vapours of bromine, gained considerably in sensitive- 

 ness. After attaining a rose tint under the influence of these vapours 

 the plate was again exposed to the vapour of iodine, until the surface 

 had gained a violet tint. Among the accelerating substances since 

 employed we may mention . chloride of iodine, several preparations of 

 bromide of iodine, of chloro-bromide of iodine, and several solutions 

 known as Hungarian liquid, German liquid, some used without the 

 aid of iodine, whilst others acted only in the wake of this chemical on 

 the surface of the silver plate. Thanks to this increase of sensitive- 

 ness in the sensitizing substances, the processes in lidioyraphy (by 

 this name Niepce from the first designated his method) were much 

 more expeditious. Views and portraits were taken in a few seconds, 

 and even the direct rays of the sun were dispensed with ; diffused 

 light sufficed for obtaining proofs, less vigorous certainly, but for that 

 reason more harmonious and more artistic. The improvements made 

 in cameras, and in optical apparatus, which will be described, have 

 also contributed to this advance. 



Before passing to the description of the photographic processes which 

 were gradually substituted for those of the first inventors, let us 

 return to the scientific physico-chemical interpretation of the pheno- 

 mena we have been studying. We have nothing to say on the purely 

 optical side of the phenomena ; the formation of the images at the 

 focus of the dark chamber has been completely explained in the 

 chapters devoted to the phenomena of light and their laws, and to 

 optical instruments, properly- so called. But what takes place on the 



