10 



Of the colours of the spectrum — red. orange, yellow, ""green, 

 blue, indigo, and violet ; those at the red end affect the photogi-aphic 

 plate more slowly, and those at the violet end more quickly than the 

 central ones ; and, when a picture cftntaining many colours has to 

 be taken, the result is always more or less unsatisfactory. Thus, 

 a young lady with blue eyes, ruby lips, pink complexion, and aubum 

 hair, is very difficult to photograph, for the eyes, if taken properly, 

 ai'e apt to be represented at the expense of other parts, while if the 

 redder tints are correctly depicted the portrait may appear without 

 any eyes at all. 



In the Talbot method, the picture which is taken on paper, is 

 what is called a '■ negative," that is to say it has reversed tints, the 

 light parts being dark, and the dark light. The negative is first 

 " developed," in order that the picture may be distinctly seen, and 

 afterwards " fixed," so that no further change may ensue under the 

 action of light. It is then applied over a sheet of sensitive paper 

 and exposed to sunlight, and as the light cannot pass through the 

 dark parts of the negative, the portions below remain white, while 

 those through which the light passes become dark ; and thus 

 a " positive " picture, the reverse of the negative, results. The 

 paper for positives is prepared by being first faced with a thin 

 coating of gelatine, or of white of egg. It is then dipped into a 

 solution of common salt in water, and afterwards di'ied. To render 

 the paper sensitive, it is necessary to float it with the coated surface 

 downwards, in a solution of nitrate of silver. It is then dried in a 

 dark room, and is ready for use. 



When the positive print has been obtained, the picture must be 

 '•' fixed," and this is done by washing it in a solution of hyposulphite 

 of soda, and thus removing all the silver salt which has not been 

 acted upon. But the picture resulting from the immediate washing 

 in the hyposulphite, is not a good one, it is too pale, and a method 

 of "toning" has been discovered, by which additional colom* of any 

 desired intensity may be introduced. This is effected by dipping 

 the print into a weak solution of acetate of soda and chloride of 

 gold. 



The Talbot plan of producing negatives was, after a time, 

 improved upon. Plates of glass were covered with films of vaiious 



