r 5 8 SCIENCE PROGRESS 



this is laid a second film of gelatin treated with bichromate of 

 potash. An image is printed on this through a reversed nega- 

 tive, in which each part is hardened in the ratio of the amount 

 of light that passes through the negative. 



To use the film for printing it must be kept at a uniform 

 degree of moisture. The hardest parts absorb none, the others 

 absorb moisture in the ratio of the action of the light on them. 

 When an inked roller is passed over the film, the ink is taken 

 up freely by the hard, unmoistened parts, while the other parts 

 take it up in inverse proportion to the moisture they hold, and 

 print off an impression exactly reproducing the values of the 

 original photograph. In the hands of an experienced printer 

 this process gives very artistic results. One great advantage of 

 the collotype is that it can be printed on any printing paper. It 

 is especially suitable for use in the cases of small editions, as the 

 prime cost of making a block is avoided. 



It is usual to obtain an estimate in this process for making 

 the collotype and printing of the reproductions together, as the 

 success depends equally on the character of the original negative 

 taken and on the skill and experience of the printer. 



Photogravure 



Photogravure is the most important of the mechanical repro- 

 ductive processes, and the most costly. A photogravure plate, 

 when completed, resembles the old mezzotint, in which the parts 

 that yield the impression are hollows in the metal, instead of 

 being ridges on it, as in line or half-tone blocks, or flat surfaces, 

 as in collotype and lithography. It is excellent for portraiture, 

 and gives the nearest approach to a facsimile reproduction in 

 black and white that has yet been obtained, chiefly because it 

 provides a mechanical basis on which an artist may work. 



In this process the first thing required is to produce a grain 

 on a highly polished copper plate, for the purpose of holding 

 the ink. This is usually done by exposing the plate to a dust- 

 cloud of bitumen, and then heating it sufficiently to attach the 

 particles to it without entirely melting them. The plate is then 

 covered by the usual film of bichromatised gelatin, and printed 

 with the subject required from a " positive" — that is, a glass film 

 in which the lights are transparent and the shadows dark. The 

 ordinary hardening process takes place, and the unhardened 

 gelatin is washed away. The plate is now bitten in with ferric 



