350 SCIENCE PROGRESS 



fall into two great classes, or, more correctly perhaps, a greater 

 class and a lesser class. The less important division includes 

 methods in which a single homogeneous surface is employed, 

 while in the larger division the surface is multiple or non- 

 homogeneous. The first is generally an attempt to get as near 

 as possible to the simple colour photography described above, 

 while the second is entirely indirect in that it does not aim 

 at producing colour at all, but only at automatically locating 

 suitable dyes or inks. We will consider the direct method first. 



Dr. T. J. Seebeck, of Jena, in the year 1810 or thereabouts, 

 while making some experiments as to the action of the solar 

 spectrum on silver chloride, observed that the violet light of 

 the spectrum produced a reddish brown substance sometimes 

 inclining towards violet, that the blue light produced a blue 

 substance, and the red a rose or lilac substance. If the silver 

 salt had previously been exposed to light for a time, so as to 

 darken it, there was a bleaching action where the red and 

 yellow light fell upon it. It is hardly likely that he had in mind 

 any photographic process whether in colour or otherwise, as 

 photography at this time was not a practical art. Eight years 

 before this Sir Humphry Davy had described at the Royal 

 Institution the attempts of Thomas Wedgwood, which he had 

 supplemented, to get a practical photographic method — a print- 

 ing process as we should now call it — but their preparations 

 were very lacking in sensitiveness, and no method of fixing 

 them was known. Of those who were primarily concerned in 

 working out practical photographic methods, the elder Niepce 

 began his work in 18 14, Daguerre in 1824, and Fox Talbot in 

 1833, and it was not until 1839 that their work culminated in 

 serviceable processes. 



Edmund Becquerel appears to have been the first to specifi- 

 cally attack the problem of the photography of colour. It is 

 stated that he began his investigations in 1838, though he did 

 not publish any results of them until ten years later. Although 

 the earlier date was just before the methods of Daguerre and 

 Talbot were published, there is no doubt that there was a 

 general knowledge in scientific circles that something of the 

 kind would shortly be made known, and that this directed 

 the attention of many investigators to the possibilities of 

 photograpny. But long before 1848 other workers had made 

 observations concerning the photography of colour. Sir John 



