1890.] on the Photographic Image. 137 



years' use of glass as a substratum, we are going back to Fox Talbot's 

 plan, and using tbin flexible films— not exactly of paper, but of an 

 allied substance, celluloid. [Specimens of Talbotypes, lent by Mr. 

 Crookes, exbibited, witb celluloid negatives by tbe Eastman Company.] 

 If I interpret tbis fragment of bistory correctly, tbe founders of 

 modern pbotograpby are tbe tbree men wbose labours bave been 

 briefly sketcbed. Tbe jubilee of last autumn marked a culminating 

 point in the work of Niepce and Daguerre, and of Fox Talbot. Tbe 

 names of tbese tbree pioneers must go down to posterity as co-equal 

 in tbe annals of scientific" discovery. [Portraits by Mr. H. M. Elder 

 shown.] The lecture theatre of the Eoyal Institution offers such 

 tempting opportunities to the chronicler of the history of this 

 wonderful art that I must close this treatment of the subject by 

 reminding myself that in selecting the present topic I had in view a 

 statement of the case of modern photography from its scientific side 

 only. There is hardly any invention associated with the present 

 century which has rendered more splendid services in every de- 

 partment of science. The physicist and chemist, the astronomer and 

 geographer, the physiologist, pathologist, and anthropologist will all 

 bear witness to the value of photography. The very first scientific 

 application of Wedgwood's process was made here by the illustrious 

 Thomas Young, when he impressed Newton's rings ^ on paper 

 moistened with silver nitrate, as described in his Bakerian Lecture 

 to the Eoyal Society on November 24, 1803. Prof. Dewar has 

 iust placed in my hands the identical slide, with the Newton rings 

 still visible, which he believes Young to have used in this classic 

 experiment. [Shown.] 



Our modern photographic processes depend upon chemical 

 changes wrought by light on films of certain sensitive compounds. 

 Bitumen, under this influence, becomes insoluble in hydrocarbon 

 oils, as in the heliographic process of the elder Niepce. Gelatine 

 mixed with potassium dichromate becomes insoluble in water on 

 exposure to light, a property utilised in the photo-etching process 

 introduced in 1852 by Fox Talbot, some of whose original etchings 

 have been placed at my disposal by Mr. Crookes. [Shown.] Chro- 

 matized gelatine now plays a most important part in the autotype 

 and many photo-mechanical processes. The salts of iron in the ferric 

 condition undergo reduction to the ferrous state under the influence 

 of light in contact with oxidizable organic compounds. The use of 

 these iron salts is another of 8ir John Herschel's contributions to 

 photography (1842), the modern " blue print " and the beautiful 

 platinotype being dependent on the photo-reducibility of these 

 compounds. [Cyanotype print developed with ferricyanide.] ^ 



Of all the substances known to chemistry at the present time, the 

 salts of silver are by far the most important in photography, on 

 account of the extraordinary degree of sensitiveness to which they 

 can be raised. The photographic image, with which it is my privi- 

 lege to deal on this occasion, is that invisible impression produced by 



