CHAP, vii.] PHOTOGRAPHY ON PAPER AND 'ON GLASS. 307 



art of drawing. We have still, however, to mention a series of dis- 

 coveries, recently made in the domain of photography, which have an 

 interest for physicists and artists. 



V. PHOTOGRAPHY WITH ARTIFICIAL LIGHT. 



Heliography, as we have seen, is founded on the property of the 

 rays of light to affect chemically those substances said to be impres- 

 sible or sensitive; it is the chemical radiation of the sun either 

 directly or in the light of day that is, diffused solar light which 

 has these photogenic properties. But the question was soon raised 

 and soon settled by physicists and photographers, whether the sun's 

 light could not be replaced at need by other light more or less 

 intense. 



The electric light, from its powerful intensity, claimed the first 

 attention. Its colouring power on chloride of silver had long been 

 known, having been demonstrated by Brande soon after the discovery 

 of the voltaic arc by Davy. M. de la Bive stated later that it acted 

 on daguerreotype plates, and this savant obtained the image of a 

 plaster bust, illuminated by the dazzling light of electricity. The 

 application of this source of light to photography is now practically 

 in use, as we find by the following notice, which we quote from Les 

 Mondes of October, 1866 : " Mr. Woodbury, of Manchester, continues 

 to employ, in the production of photographic negatives on gelatine, 

 the electric light produced by Wilde's machine. This light, produced 

 between the points of two pieces of carbon, is surrounded by nega- 

 tives which it must penetrate to impress the gelatine; and we 

 maintain that the reliefs on gelatine obtained with the electric light 

 are better defined than when obtained with sunlight or daylight." 

 This notice evidently relates to a process of heliographic engraving 

 which we shall mention later ; but what follows relates to the pro- 

 duction of real photographic negatives : " Messrs. Saxon and Co., also 

 of Manchester, use now, exclusively, Wilde's electric light in enlarging 

 photographs. In possession of an artificial light which shines day 

 and night, they are enabled to undertake to enlarge, in twenty-four 

 hours, the negatives entrusted to them." 



This light is however veiy costly. In the rare cases when photo- 

 graphy at night is necessary, a preference is given to the light 



x 2 



