THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 289 



The varjii.sh may be applied iiidilTerently to inetaLs, stone, or glass, 

 but Monsieur Niepce prelers copper plated with silver. To take 

 copies of engravings, a snuill (piantity of wax is dissolved in essential 

 oil of lavender and added to the varnish already described. The en- 

 graving, first varnished over the back, is placed on the surface of the 

 prepared tablet, face toward it, and then exposed to the action of the 

 light. In the camera obscura an exposure of from six to eight hours, 

 varying with the intensity of the ligfit, is required; while from four 

 to six hours is necessary to produce a copy of an engraving. The 

 picture, in the first instance, is represented by the contrast between 

 the polished silver and the varnish coating. The discoverer after- 

 wards adopted a plan of darkening the silver by iodine, which ap- 

 pears to have led the way to Daguerre's beautiful process. To darken 

 the tablet, it Avas placed in a box in wdiich some iodine was strewed 

 and watched until the best eft'ect was produced. The varnish was 

 afterwards removed by spirit of wine. 



Of the use of glass plates Monsieur Niepce thus speaks: 



Two experiments in landscape upon glass, by means of the camera, gave me 

 results which, although imperfect, appear deserving of notice, because this 

 variety of application may be brought more easily to perfection, and in the end 

 become a more interesting department of heliography. 



In one of these trials the light acted in such a way that the varnish was 

 remove<l in proportion to the intensity with which the light had acted, and the 

 picture exhibited a more marked gradation of tone, so that, viewed l\v transmit- 

 ted light, the landscape produced, to a certain extent, the well-known effects of 

 the diorama. 



In the second trial, on the contrary, the action of the luminous tiui<l having 

 l)een more intense, the parts acted upon by the strongest lights, not having 

 been attacked by the solvent, remained transparent. The difference of tone 

 resulted from the relative thickness of the coatings of varnish. 



If this landscape is viewed by reflection in a mirror on the varnished side 

 and at a certain angle, the effect is remarkably striking, while, seen by a trans- 

 initted light, it is confused and shapeles.s, but what is equally surprising, in this 

 position the mimic tracery seems to affect the local color of the objects. 



A statement that M. NiejDce was enabled to engrave by light went 

 the round of the press: but this does not appear to have V)een the 

 (•ase. All that the author of heliography effected was the etching of 

 the plate, after it had tmdergone its various processes, and the draw- 

 ing was completed by the action of nitric acid in the usual manner. 

 The parts of the copperplate protected by the varnish remained, of 

 course, unacted on, while the other parts were rapidly attacked by the 

 acid. JNiepce remarks that his process can not be used during the 

 winter season, as the cold and moisture render the varnish l)rittlc and 

 detach it from the glass or metal. 



M. Niepce afterwards used a more unctuous \'arnish, composed of 

 bitumen of Judea dissolved in animal oil of Dippel. This composi- 

 bM 1901 19 



