154 Photography — Its History, Position, and Prospects. 



short general view of its most important processes ; to explain 

 the most interesting 'of the principles on which its manipula- 

 tions are founded ; and finally to notice some of the accidents 

 and failures to which it is liable. Although it is almost im- 

 possible to keep these perfectly distinct, it is more convenient 

 to treat them separately. 



Photography (light writing) enables us to produce pictures 

 by means of the sun's rays, nevertheless its designation has 

 not been happily chosen. Niepce very early suggested the 

 term Heliograplty (sun writing), which would be far less open to 

 objection ; but it was not adopted : and the name, like many 

 others which arc unfortunately found in science, and were formed 

 under the dangerous guidance of imperfect knowledge, is 

 likely to maintain its position. It is not improbable, however, 

 that it may yet be appropriate, since the day appears not very 

 remote when even the various coloured rays will be compelled 

 to leave their characteristic and permanent impress, and the 

 photograph will become a picture in the truest sense of the 

 word. 



Histoky of Photography. Photography originated in, and 

 its chief processes still arc — as perhaps they always will be — 

 founded on the fact that the salts of silver are blackened by light. 

 This, although so recently utilized, is not a new discovery. So 

 early as the middle of the fifteenth century the alchymists had 

 observed the blackening of fused chloride of silver; and they 

 even considered the " sulphurous principle " of light, as they 

 termed it, one of the chief agents through which nature re- 

 ceived her variety of form. This extraordinary property of 

 light continued, at least from time to time, to arrest the atten- 

 tion of philosophers, but the progress of its investigation was 

 long and tedious. In 1777, Scheele concluded from his experi- 

 ments that the dark tint produced was due to "reduced sil- 

 ver" ; and he remarked that the violet acted more energetically 

 Ilia n any other ray. In 1801, Bitter observed that a silver 

 salt was lihukeiied in a space beyond the violet of the spectrum, 

 and that the red ray restored the reduced chloride. Then, it 

 was discovered that the action of light was not confined to 

 argentiferous compounds; Wbllaston, in 1802, ascertained that 

 cards moistened with tincture of gum guiaiacum acquired a 

 green tint in the violet ray, but lost it in the red. In 1810, 

 Seebecfc noticed thai the tints produced by light on chloride 

 of silver were different with the different coloured rays; violet 



rendering it violet ; blue, blue; yellow, white; and red, veil. 

 This was the first approximation to Heliochromy. I'orard per- 

 ceived that, when tin; rays of the spectrum, from the green to 

 tin' extremity of the red, were coneeiit rated by a lens, chlo- 

 ride of silver, exposed in the focus for more than two hours, 



