i 5 6 SCIENCE PROGRESS 



small white spaces among them, we get shadows; when they 

 are small we get lights. This effect is produced by intro- 

 ducing into the camera, just in front of the sensitive film, a glass 

 plate cross-ruled in parallel lines from i/5oth to i/20oth of an 

 inch apart. 



As the picture from the lens passes through the little squares 

 of the ruled screen, thousands of minute cones of light are pro- 

 duced. Where the bright part of the picture falls on the sensitive 

 plate there is greater action, where the darkest parts fall there is 

 often very little, and thus some of these minute spots of altered 

 silver in the negative may meet each other on the sensitive film, 

 while others do not. In this way the varying size of the white 

 spaces which produce the half-tones is obtained. 



The negative, when ready, is printed on a highly polished 

 piece of copper which is coated with fish-glue treated with 

 bichromate of ammonia, and the unaltered parts of the print 

 are washed away as in the case of the line block. The plate 

 with its gelatin picture is heated until the lines and dots, now 

 insoluble in acid, adhere firmly to the metal, and is then placed 

 in a bath of ferric chloride, which etches away the unprotected 

 surface and leaves the lines and dots standing free. The first 

 etching is a rough one, and, to bring out contrast, it is usually 

 necessary to re-bite portions of the plate several times, the 

 remainder being protected from the bath by coats of varnish. 

 In cases where special care is required, handwork engraving on 

 the block is resorted to, but this adds greatly to the expense. 



Photographs of every kind, sepia sketches, black and white 

 wash drawings, chalk drawings, line engravings, mezzotints, and 

 photogravures can be reproduced to great advantage by this 

 process. A pencil drawing hardly ever makes a good reproduc- 

 tion. The chief objection to the half-tone block is that it cannot 

 reproduce the actual range of the artist's expression. A pure 

 white is impossible without engraving on the block, and light 

 tints are degraded by the screen lines, robbing the print of 

 brilliancy. 



When an oil-painting or water-colour sketch is to be repro- 

 duced, a new difficulty arises. Colours affect photographic 

 plates in a very different way from that in which they strike 

 the eye. The yellow in a picture will come out as black, while 

 the blue will act as a white would. To obviate this difficulty, 

 either special plates which give approximately equal values to 



