288 THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY 



dry in a gentle temperature and protected from a damp atmosphere. 

 In this part of the operation a light disk of metal with a handle in 

 the center should be held before the mouth in order to condense the 

 moisture of the breatli." 



The plate thus ]:)i-e])ared is now in a fit state for use and may be 

 immediately fixed in the correct focus of the camera. After it has 

 been exposed a sufficient length of time for receiving the impression 

 a very faint outline alone is visible. The next operation is to bring 

 out the hidden picture, which is accomplished by a solvent. 



This solvent must be carefully adapted to the purposes for which it 

 is designed. It is difficult to fix Avith certainty the proportions of its 

 components, but in all cases it is better that it be too weak than too 

 strong. In the foi'mer case the image does not come out strongly ; in 

 the latter it is completely destroyed. The solution is prepared of one 

 part — not by weight, but volume — of the essential oil of lavender, 

 poured upon ten ])arts, l)y measure also, of oil of white petroleum. 

 The mixture, which is first milky, becomes clear in two or three days. 

 This compound will act until it becomes saturated with the asphaltum, 

 which is readily distinguished by an ()i)a(iue appearance and dark 

 brown color. A tin vessel somewhat larger than the photographic 

 tablet and 1 inch deep must be provided. This is to have as nuich 

 of the solvent in it as Avill cover the plate. The tablet is plunged 

 into the solution, and the operator, ol)serving it by reflected light, 

 begins to see the images of the objects to which it has been exposed 

 slowly unfolding their forms, though still veiled by the gradually 

 darkening supernatant fluid. The j^late is then lifted out and held 

 in a vertical j)osition till as much as possible of the solvent has been 

 allowed to drop away. Wlien the dropping has ceased, we jjroceed to 

 the last, and not the least important, operation of washing the plate. 

 This is performed by carefully placing the tablet upon a board B (fig. 

 1), fixed at a large angle in the trough A A, the supports being joined 



to it by hinges to admit of the neces- 

 sary changes of inclination under 

 different circumstances. Two small 

 blocks, not thicker than the tablet, 

 are fixed on the board on Avhich the 

 plate rests. Water must now be 

 slowly i)oured upon the upper part 

 of the board and allowed to flow 

 evenly over the surface of the pic- 

 1 ture. The descending stream clears 



away all the solvent that may yet 

 adhere to the varnish. The plate is now to be dried with great care 

 by a gentle evapoi-ation. To preserve the i)icture it is requisite to 

 cover it up from the action of light and jjrotect it from humidity. 



