380 THE PHOTOGRAPHIC IMAGE. 



phy. He was the first to produce a transparent paper negative from 

 which any number of positives could be obtained by printing through. 

 The silver print of modern times is the lineal descendant of the Tal- 

 botype i>riut. After 40 years' use of glass as a substratum we are 

 going back to Fox Talbot's plan, and using thin flexible films — not ex- 

 actly of paper, but of an allied substance — celluloid. [Specimens of 

 Talbotypes, lent by Mr. Crookes, exhibited, with celluloid negatives by 

 the Eastman Company.] 



Tf I interpret this fragment of history correctly, the founders of mod- 

 ern photography are the three men whose labors have been briefly 

 sketched. The jubilee of last autumn marked a culminating point in 

 the work of Niepce and Daguerre and of Fox Talbot. The names of 

 these three pioneers must go down to posterity as coequal in the annals 

 of scientific discovery. [Portraits by Mr. H. M. Elder shown.] The 

 lecture theater of the Royal Institution oflers such tempting opportu- 

 nities to the chronicler of the history of this wonderful art that I tnust 

 close this treatment of the subject by reminding myself that in select- 

 ing the present topic I had in view a statement of the case of modern 

 photography from its scientific side only. There is hardly any inven- 

 tion associated with the present century which has rendered more splen- 

 did services in every department of science. Tlie physicist and chemist, 

 the astronomer and geographer, the physiologist, pathologist, and an- 

 thropologist 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 Royal Society on November 24, 1803. Professor Dewar has just 

 placed in my hands the identical slide, with the Newton rings still visi- 

 ble, 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 hydro-carbon oils, as in the 

 heliographic process of the elder Niepce. Gelatine mixed with potas- 

 sium dichromate becomes insoluble in water on exposure to light, a 

 property utilized in the photo-etching process introduced in 1852 by 

 Fox Talbot, some of whose original etchings have been placed at ray 

 disposal by Mr, (3rookes. [Shown.] Chromatized gelatine now plays 

 a most important part in the autotype and many photo-mechanical proc- 

 esses. The salts of iron in the ferric condition undergo reduction to the 

 ferrous state under the influence of light in contact with oxidizable or- 

 ganic compounds. The use of these iron salts is another of Sir 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.] 



