PHOTOGKAPHY ON GLASS. 89 



rializing, and place it upon a sheet of glass prepared as follows. 

 Cut a piece of thin plate-glass to the size of the daguerreotype, 

 and affix to one side, with gum, a thin oval piece of blackened 

 zinc, so that the centre of the oval shall correspond with the 

 centre of the image upon the plate. Having carefully placed 

 the glass, thus prepared, with the centre of the zinc disc upon 

 the centre of the image, expose the whole to daylight for 20 

 seconds. The action of the light will obliterate all traces of the 

 image from every part of the plate, except that which is co- 

 vered with the blackened zinc. The thickness of the glass will 

 also cause the action to be refracted under the edges of the 

 zinc disc, and will soften into the dark parts. Mercurialize 

 the plate as usual ; the image will be found with a halo of 

 light around it, gradually softening into the black ground, 

 that will at once add a new charm to these interesting produc- 

 tions. By grinding the glass on which the disc is fixed, and 

 by altering the size and shape of the disc, a variety of effects 

 may be produced, which every ingenious operator can suggest 

 for himself." 



Photograpliy on Glass. — See Niepce's communication in 

 Comptes Rendus, and an extract from the same in Lond. 

 Journ. Oct. 1850. 



Photogenic Glasses. — Ceselli (Athenseum, 1850) gives the 

 following direction for albumizing glass plates, so as to pro- 

 duce a perfectly uniform and smooth surface. 



The requisite apparatus consists of a small rectangular box 



supported by three regulating screws. To its base is joined a 



movable plate of metal, which, being heated by a spirit lamp, 



communicates to all parts of the box an equal degree of heat. 



The plate is removed when the water-bath is to be used instead 



of the lamp. The apparatus is protected by a glass covering, 



to guard against heterogeneous bodies falling on the albumen. 



This cover is also movable ; and the box being traversed by 



an internal channel, in this, when convenient, a thermometer 



may be introduced. A sliding frame receives the glass that 



is to be coated ; this again being placed between two other 



])lates of glass. The glasses are secured and their edges 

 b2 



