248 



NATURE 



[July 10, 1890 



films — not exactly of paper, but of an allied substance, 

 celluloid. [Specimens of Talbotypes, lent by Mr. Crookes, 

 exhibited, with celluloid negatives by the Eastman 

 Company.] 



If I interpret this fragment of history correctly, the 

 founders of modern photography are the three men 

 whose labours 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 co- 

 equal in the annals of scientific discovery. [Portraits by 

 Mr. H. M. Elder shown.] The lecture theatre of the 

 Royal 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 department of science. 

 The physicist and chemist, the astronomer and geo- 

 grapher, the physiologist, pathologist, and anthropo- 

 logist will all bear witness to the value of photography. 

 The very first scientific application of Wedgwood's pro- 

 cess 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. Prof. Dewar 

 has just 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 be- 

 comes insoluble in hydrocarbon oils, as in the helio- 

 graphic process of the elder Niepce. Gelatine mixed 

 with potassium 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 my dis- 

 posal by Mr. Crookes. [Shown.] Chromatized 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 Sir John Herschel's contributions to photo- 

 graphy (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 photo- 

 graphic image with which it is my privilege to deal on 

 this occasion is that invisible impression produced by the 

 action of light on a film of a silver haloid. Many methods 

 of producing such films have been in practical use since 

 the foundation of the art in 1839. AH these depend on 

 the double decomposition between a soluble chloride, 

 bromide, or iodide, and silver nitrate, resulting in the 

 formation of the silver haloid in a vehicle of some kind, 

 such as albumen (Niepce de St. Victor, 1848) or collodion 

 on glass, as made practicable by Scott Archer in 1851. 

 For twenty years this collodion process was in universal 

 use ; its history and details of manipulation, its develop- 

 ment into a dry plate process by Colonel Russell in 1861, 

 and into an emulsion process by Bolton and Sayce in 

 1864, are facts familiar to everyone. 



The photographic film of the present time is a gelatino- 

 haloid (generally bromide) emulsion. If a solution of 

 silver nitrate is added to a solution of potassium bromide 

 and the mixture well shaken, the silver bromide coagulates 



NO. T080, VOL. 42] 



and rapidly subsides to the bottom of the liquid as a 

 dense curdy precipitate. [Shown.] If instead of water we 

 use a viscid medium, such as gelatine solution, the 

 bromide does not settle down, but forms an emulsion, 

 which becomes quite homogeneous on agitation. [Shown.] 

 This operation, omitting all details of ripening, washing, 

 &c., as well known to practical photographers, is the basis 

 of all the recent photographic methods of obtaining 

 negatives in the camera. The use of this invaluable vehicle, 

 gelatine, was practically introduced by R. L. Maddox in 

 1 87 1, previous experiments in the same direction having 

 been made by Gaudin (1853-61). Such a gelatino- 

 bromide emulsion can be spread uniformly over any sub- 

 stratum — glass, paper, gelatine, or celluloid — and when 

 dry gives a highly sensitive film. 



The fundamental problem which fifty years' experience 

 with silver haloid films has left in the hands of chemists 

 is that of the nature of the chemical change which occurs 

 when a ray of light falls on such a silver salt. Long 

 before the days of photography— far back in the sixteenth 

 century — Fabricius, the alchemist, noticed that native 

 horn silver became coloured when brought from the mine 

 and exposed. The fact presented itself to Robert Boyle 

 in the seventeenth century, and to Beccarius, of Turin, in 

 the eighteenth century. The change of colour undergone 

 by the chloride was first shown to be associated with 

 chemical decomposition in 1777 by Scheele, who proved 

 that chlorine was given off when this salt darkened under 

 water. I can show you this in a form which admits of its 

 being seen by all. [Potassium iodide and starch paper 

 were placed in a glass cell with silver chloride, and the 

 arrangement exposed to the electric light till the paper 

 had become blue.] The gas which is given off under 

 these circumstances is either the free halogen or an oxide 

 or acid of the halogen, according to the quantity of 

 moisture present and the intensity of the light. 1 have 

 found that the bromide affects the iodide and starch paper 

 in the same way, but silver iodide does not give off any 

 gas which colours the test paper. All the silver haloids 

 become coloured on exposure to light, the change being 

 most marked in the chloride, less in the bromide, and 

 least of all in the iodide. The latter must be associated 

 with some halogen absorbent to render the change visible. 

 [Strips of paper coated with the pure haloids, the lower 

 halves brushed over with silver nitrate solution, were 

 exposed.] The different degrees of coloration in the three 

 cases must not be considered as a measure of the relative 

 sensitiveness : it simply means that the products of 

 photo-chemical change in the three haloids are inherently 

 possessed of different depths of colour. 



From the fact that halogen in some form is given off, 

 it follows that we are concerned with photo-chemical 

 decomposition, and not with a physical change only. All 

 the evidence is in favour of this view. Halogen ab- 

 sorbents, such as silver nitrate on the lower halves of the 

 papers in the last experiment, organic matter, such as the 

 gelatine in an emulsion, and reducing agents generally, 

 all accelerate the change of colour. Oxidizing and 

 halogenizing agents, such as mecuric chloride, potassium 

 dichromate, &c., all retard the colour change. [Silver 

 chloride paper, painted with stripes of solutions of sodium 

 sulphite, mercuric chloride, and potassium dichromate, 

 was exposed.] It is impossible to account for the action 

 of these chemical agents except on the view of chemical 

 decomposition. The ray of light falling upon a silver 

 haloid must be regarded as doing chemical work ; the 

 vibratory energy is partly spent in doing the work of 

 chemical separation, and the light passes through a film 

 of such haloid partly robbed of its power of doing similar 

 work upon a second film. It is difficult to demonstrate 

 this satisfactorily in the lecture-room on account of the 

 opacity of the silver haloids, but the work of Sir John 

 Herschel, J. W. Draper, and others has put it beyond 

 doubt that there is a relationship of this kind between 



