IK) J. Waterhouse — The Application of Fhofograpliy [No. 2, 



The plate is then etched in the usual way and rolled up with a varnish 

 ink, containing a large proportion of resinous matter ; it is then dusted with 

 powdered resin, which sticks to the lines and renders them more capable of 

 resisting the acid ; the superfluous resin is brushed off and the plate is gently 

 heated. 



The edges of the plate and the large white spaces are covered with 

 shellac varnish and Avhen the varnish is thoroughly dry, the plate is plunged 

 into a trough containing very weak dilute nitric acid, kept in constant 

 motion, and is left until the finest parts are sufficiently bitten, which 

 generally takes about a quarter of an hour ; it is then taken out of the 

 trough, washed, dried, and placed on a sort of grating over a charcoal fire. 

 Under the influence of the heat, the coating of ink and resin on the 

 lines, being gently softened, flows down and protects the sides of the 

 hollows formed by the first biting, filling up the spaces where the lines 

 are very close. As soon as this effect is produced the plate is allowed 

 to cool and then inked with a lithographic roller, as if a proof was going to 

 be pulled. It is again dusted with powdered resin, and is then ready for a 

 second biting in, which is to attack the parts somewhat lighter, and there- 

 fore may be effected with stronger acid. 



The operations of inking, dusting with resin, heating and biting with 

 acid are repeated several times till the plate presents only a uniform black 

 colour. Tlien the plate is bitten with strong dilute acid which bites out the 

 parts to be left completely white. The large whites, which have been 

 covered all along with a strong shellac varnish, are then cut out with a saw, 

 and the plate is ready to be mounted on a wooden or leaden block for print- 

 ing. These plates usually require considerable touching up to take off the 

 ragged edges of the lines caused by the spreading of the ink, though this may 

 also be done by repeating the inking and biting in, so as to remove the 

 steps formed by the successive bitings.* 



This process has been applied at the Imprimerie Nationale, Paris, for 

 producing large geological maps, but the special precautions that had to be 

 taken in " overlaying" the plates in the press so as to print properly were 

 very tedious, and must have largely increased the exj)ense and lessened the 

 use of the process. Messrs. Yves and Barret, of Paris, are said to use it 

 largely for reproducing maps and engineers' plans, &e. 



FJiolo-hloclcs in haJf-tone. — Many attempts have been made from 

 time to time to obtain surface blocks from photographs from nature and 

 other shaded subjects, but with imperfect success. If this object could be 

 successfully attained, it is easy to understand that it would be of im- 

 mense value for book and newspaper illustration and many other purposes. 

 There are, unfortunately two grave difliculties to be overcome — one caused 

 * See Diivannc, ' Les Frogres de la Fhotographiey p. 201. 



