THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 301 



mercury and iodiiu'. The plate niny now be deposited in the grooved 

 box (tig. 11), in whicli it may be Icept, excluded from the light, 

 until it is convenient to perform the last fixing operation. 



Fifth operation.— Th'i^ process has for its object the removal of 

 the iodine from the plate of silver, which prevents 

 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 

 Avell as a Aveak solution of the hyposulphite of soda. 

 In the first place the plate is to be placed in a trough 

 of water, plunging and withdrawing it immediately ; 

 it is then to be plunged into one of the above saline 

 solutions, which would act upon the drawing if it 

 was 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. AMien 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 

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

 drawings. This finishes the draAving, 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 rubbing. To preserve 

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

 glass coA^er over them and then framed in Avood. They are now 

 unalterable by the sun's light. 



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

 vided the silver Ix- not polished through to the copper. It is very 

 important, after each trial, that the mercury be remoA'ed immedi- 

 atelv by polishing \Tith pumice poAvder and oil. If this ()e neg- 

 lected the mercury finally adheres to the silver, and good drawings 

 can not be obtained if this anuilgam is present. 



D. The Calotype. 



Early in 1840 draAvings on paper were handed about in the 

 scientific circles of London and of Paris, Avhich Avere a great advance 

 upon anything Avhich had been previously done. These w^ere the 

 results of a ncAv process discovered by Mr. Talbot, and then at- 

 tracted so much attention that Monsieur Biot made them the subject 



