538 PROCEEDINGS OP SECTION I. 



influence in works of art — if we place a bright silver coin upon 

 the table and centre the eye upon it for a few minutes, and then 

 remove the coin for an instant, there is a white spot in its place 

 which almost immediately seems black. This I believe, is ex- 

 plained by supposing the nerve to be so exhausted by the 

 continued action of the special rays that it momentarily becomes 

 insensible to their influence, and the effect of the rays of an 

 opposite kind is transmitted to the brain. 



We know the highest part of the tower is in the brightest light, 

 but it does not appear so to the eye. The reason is obvious when 

 we look at a tower or a church steeple, a great part of the retina 

 is opposed to the light of the sky, and on shifting the eye to any 

 particular point, the light which is reflected from the part falls 

 upon the retina where it has been exhausted by the direct light 

 from the sky, and consequently that part appears dark. ' 



This peculiarity of the eye is taken advantage of by painters, 

 and they create an eff^ect not only by the contrasts of light and 

 shade, but also by contrast of colour, and in the application of 

 photography to re-produce pictures, the faults of photography have 

 been, to some extent, corrected by the artist's final touches and 

 by the use of gelatine films and shading mediums. To these I 

 will refer later. 



The methods of automatic and mechanical engraving are, in 

 trade parlance, described as ^^ process work f these processes are 

 either chemical or photographic. 



It is not more than fifty years since, that for all purposes of 

 book and newspaper illustration, wood engraving was supreme. 

 In a treatise on wood engraving, published in 1839, the authors 

 claimed that wood engraving had then practically superseded 

 copperplate engraving, which for centuries preceding had held 

 sway. 



More recently it has been said that by the aid of photography 

 modern processes would supersede wood engi-aving, but so far this 

 has not been effected, and better copperplate engraving and more 

 beautiful wood engraving are now produced than ever, each art 

 being stimulated by the competition that has arisen. 



The earliest form of automatic engraving was etching, which, 

 properly speaking, does not mean biting a picture by means of 

 acid on to a plate, but first drawing and then fixing the picture on 

 the plate which is to be bitten — a clever mechanic can manage the 

 biting process — it requires an artist to etch the picture. 



Chemical engraving or etching was practised in England forty 

 years ago, for at tlie Great Exhibition in 1851 specimens of work 

 were shown under the names of " Chemitypy " and " Panicono- 

 graphy." 



Of all chemical processes, Zincography is the best known in 

 England, it obtains its name from the zinc plates which are used 

 for printing the designs. It was invented by a M. Gillot, of Paris, 



