724 CATALOGl'l-: OF GRAPHIC ART EXHIBIT. 



is characteristic of the mauy minor processes illustrated in Part I, 

 until in nature-priuting nature is made to do the whole of the work^ 

 at least for simple representations of certain classes of t)bjects. In this 

 l)rogression tlie «.'n.uraver is first made supertinous, and finally the 

 designer is also discharged. 



This desire received a fresh impetus from, or rather led to. the in- 

 vention of sun })icture-making, or i)hotography. Photography itself 

 is outside of tiie scope of the present exhibition and is dealt with in 

 another department. It is a purely photo- chemical process, from the 

 first making of the image to its multiplication by the repeated action 

 of the sun's rays. The processes here to be considered are photo-mechan- 

 ical ; that is to say, while the block or plate which is to produce the 

 print is wholly or i>arrly the result of the chemical action of light, 

 the print itself is the product of the press, which is a mechanical con- 

 trivance. 



The possibility of producing a [)rintable plate or block by means of 

 photography, without the intervention of the engraver, rests upon the 

 efifect exercised by light upon certain substances, princiiially asphal- 

 turn and gelatine. The former is rendered insoluble iu its ordinary 

 solvents by the action of light. It follows that a metal plate covered 

 with asphaltum and exposed to the light in certain parts while it is pro- 

 tected from it in others, can be etched like a plate i)rei)ared for etching 

 in the ordinary manner. Gelatine, according to the process used, can 

 be hardened in those parts exposed to the light, while the others retain 

 the faculty of swelling under the effect of moisture; or it may be so 

 treated that the unexposed parts can be washed away, while those ex- 

 posed remain insoluble; or the affinity which it has, when suitably 

 treated, for fatty inks can also be made use of. Finally, it can be made 

 indifferent to acids, and can then be used iu etching processes. It is 

 quite impossible to enter here upou the details of the many processes 

 based upon these facts, most of which are, indeed, to a certain extent 

 secret. The results, however, are fully illustrated in the exhibition. 

 To gain a better idea of their peculiarities, these processes may be 

 classed i:i two ways — first, as to the kind of block or plate produced; 

 secondly, as to the sort of original used. 



As to the blocks and plates produced, they are again divisible into 

 two large and very distinct general classes, those grouped in the first 

 resembling in their main features the blocks and plates produced by 

 the old hand processes, viz: (1) Relief blocks for the type press ; (2) 



