THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 301 



mercury and iodine. The plate may now be deposited in the i^rooved 

 box (fig. 11), in Avhich it may be kept, excluded from the light, 

 until it is convenient to perform the last fixing operation. 



Fifth operation. — This process has for its object the removal of 

 the iodine from the plate of silver, which i)re vents 

 the further action of the light. 



A saturated solution of common salt may be used 

 for this purpose, but it does not answer nearly so 

 well as a weak solution of the hyposulphite of soda. 

 In the first place the plate is to l)e placed in a trough 

 of water, plunging and withdrawing it inunediately ; 

 it is then to be plunged into one of the al)ove saline 

 solutions, which would act upon the drawing if it 

 11. Avas not previously hardened by washing in water. 



To assist the effect of the saline washes, the plate 

 must be moved to and fro, which is best done by passing a wire be- 

 neath the plate. ^Mien the yellow color has quite disappeared the 

 plate is lifted out. great care being taken that the impression is not 

 touched, and it is again plunged into water. A vessel of warm dis- 

 tilled water, or very pure rain water boiled and cooled, being pro- 

 vided, the plate is fixed on an inclined plane and the water is poured 

 in a continuous stream over the picture. The drops of water which 

 may remain upon the plate must be removed by forcibly blowing 

 ui^on it, for otherwise, in drying, they would leave stains on the 

 drawings. This finishes the drawing, and it only remains to pre- 

 serve the silver from tarnishing and from dust. 



The shadows in the daguerreotype pictures are represented by 

 the polished surface of the silver, and the lights by the adhering 

 mercury, which will not bear the slightest rubljing. To preserve 

 these sketches they must be placed in cases of pasteboard with a 

 glass cover over them and then framed in wood. They are now 

 unalteral)]e by the sun's light. 



The same plate may be employed for many successive trials, pro- 

 vided the silver be not polished througii to the copper. It is very 

 important, after each trial, that the mercury be removed immedi- 

 ately by polishing with pumice powder and oil. If tliis l)i> neg- 

 lected the mei'cury finally adheres to the silver, and good drawings 

 can not be obtained if this amalgam is j^iesent. 



D. The Calotype. 



Early in 1840 drawings on paper were handed al)()ul in the 

 scientific circles of London and of Paris, which were a great advance 

 upon anything which had b-UMi previously done. These were the 

 results of a new pi-ocess discovered by Mr. Talbot, and (lien at- 

 tracted so much attention that Monsieur Biot made them tlie subject 



