PHOTOGRAPHY. 



employed by artists for this purpose is either 

 common turpentine varnish mixed with a little 

 white-lead, or a strong aqueous solution of isin- 

 glass. In performing the process on a small 

 scale, purified bees-wax will be found to form a 

 convenient varnish. 



' The piece of glass to be etched is first of all 

 warmed in a convenient manner, and one of its 

 surfaces is then rubbed over with wax, the tem- 

 perature of the glass being high enough to cause 

 the wax to melt, and be distributed uniformly 

 over the entire surface. The glass is then set 

 aside to cool ; and when the wax has become 

 quite solid, the design may be traced with a pointed, 

 but not very fine instrument, such as a bodkin. 

 A carpenter's brad-awl is a convenient tool for 

 this operation, since, from being flattened at the 

 end in one direction, it may be made to trace 

 lines of different degrees of fineness according to 

 the position in which it is held. Care must be 

 taken to cut through the entire thickness of the 

 wax, so as to lay the glass quite bare through the 

 whole length of the line. 



'The next part of the process consists in the 

 application of the hydrofluoric acid. The vessel 

 employed for this purpose is a shallow basin, 

 either of lead or of Wedgwood's ware no glazed 

 vessel should be employed large enough to in- 

 clude within its area every part of the design, when 

 the prepared glass is placed upon its edge. The 

 materials for generating hydrofluoric acid consist- 

 ing of one part of powdered fluor-spar, and about 

 two parts of highly concentrated oil of vitriol are 

 introduced into the basin, and well mixed ; the 

 glass-plate is then laid on the edge of the basin, 

 with the waxed side undermost, and a moderate 

 heat is applied, to disengage the vapour of hydro- 

 fluoric acid. A spirit-lamp will be found a con- 

 venient source of heat, from the facility it affords 

 of increasing or decreasing the temperature at 

 pleasure. Care must be taken to prevent the heat 

 becoming so powerful as to melt the wax-ground. 



'After being exposed to the acid fumes for a 

 few minutes, the glass-plate may be removed and 

 cleaned. The lines where the wax had been 

 removed are found to be covered with a white 

 powder, which consists of silico-fluorides of the 

 metallic bases of the glass. The greater part of 

 the wax may be removed by scraping with a 

 common table-knife, and the remainder by warm- 

 ing the glass before the fire, and wiping it with 

 tow and a little oil of turpentine. The design will 

 then be found to be perfectly etched upon the 

 surface of the glass, the depth of the lines being 

 proportional to the time the glass was exposed to 

 the acid vapour. In conducting the operation, 

 care should be taken not to allow the hands to 

 become exposed to the acid fumes, as the vitality 



A recent invention, called the Sand-blast, has 

 been used very successfully in engraving on glass 

 The design is either cut out in stencil, and pasted 

 on the glass, or, for more delicate work, the whole 

 surface is covered with some softer substance, as 

 collodion, and the parts wanted to be engraved 

 treated in the manner described above for etching 

 *me sand is then forced against the glass by 

 means of a blast of air, or steam, through a very 

 small opening, and is found to cut very rapidly 

 into the surface where it is not protected. This 

 process promises to be of very great importance 

 m the ornamentation of glass. 



GLYPHOGRAPHY. 



This process, the invention of Mr Palmer, is 

 essentially the following : An ordinary plate of 

 copper is stained black, and then coated with a 

 substance like white wax. In this coating, the 

 drawing is made as if for etching, only not re- 

 versed; and the remaining portions, which con- 

 stitute the 'lights' of the drawing, being height- 

 ened in various degrees by chemical deposition, a 

 cast of copper is finally deposited on the surface 

 by the electrotyping process. This, when removed, 

 shews the drawing in relief, and can now be 

 printed from. The glyphographic process is 

 applied with success to the production of maps. 



of the parts would be instantly destroyed by the 

 action of the acid.' According to the authority 

 now quoted, a dilute aqueous solution of hydro- 

 fluoric acid may be substituted for the vapour in 

 the above process with a similar result. 



Glass-plates engraved upon in this manner are 

 not adapted for printing, or yielding impressions 

 upon paper, on account of their brittleness ; but 

 even this has been partially obviated by a German 

 invention, wherein the glass, of considerable 

 thickness, is cemented to blocks of wood the 

 plates so mounted enduring sufficient pressure to 

 produce a fair impression without fracture. 



PHOTOGRAPHY. 



The word photography is derived from two 

 Greek words, and means, literally, ' writing or 

 drawing by light,' and is applied to all the various 

 methods by which images can be produced by 

 the action of light on chemically prepared sur- 

 faces. 



Almost all substances are more or less affected 

 by light, but the salts of silver, and in a lesser 

 degree, the salts of chromium, have hitherto 

 formed the basis of photography. In the i6th 

 century, the alchemists noticed that horn-silver 

 (the fused chloride) was blackened by light, but 

 no attempt to turn the discovery to any practical 

 account was made till 1802, when Wedgwood and 

 Davy succeeded in obtaining images on leather 

 and paper, which had been covered with that 

 substance, or moistened with a solution of silver 

 nitrate. When such a prepared surface was 

 covered by an engraving, and exposed to the 

 action of light, the parts under the transparent 

 paper rapidly blackened, while the parts under 

 the ink lines, through which no light could pass, 

 remained white. In this way, a reversed, or 

 negative picture was obtained, in which the light 

 parts of the engraving were represented by dark 



parts, and vice versa. Of course it will be evident 

 that if a second sheet of prepared paper were ex- 

 posed under the negative so obtained, a positive 

 picture, or one in which the lights and shades arc 

 in their proper positions, would be produced. 



The experiments of Wedgwood and Davy, 

 however, led to no practical result, as they did 

 not succeed in removing the unaltered chloride 

 of silver, after the image was produced, and, in 

 consequence, the whole surface blackened on 

 exposure to light ; nor could they obtain any 

 result in the camera-obscura, their prepared sur- 

 faces not being sufficiently sensitive to be affected 



