94 J. Watorho\isc — The Applicdt'ion of Plotograplry [No 2, 



removed, and the plates are ready for exposure under the negative, which 

 must be a reversed one obtained as described in section III. 



If the reversed negative has been taken direct on glass, the exposure 

 to light is performed in a pressure frame, in the same way as for ordinary 

 photographs. It is advisable, however, to secure clean margins by shielding 

 the borders of the negative by means of a mask, cut out in yellow or brown 

 paper, which should well overlap the edges of the printing plates. The 

 sensitive plate may be rubbed over with a little powdered soapstone to pre- 

 vent any adherence to the negative. Some sheets of dark-coloured paper or 

 cloth should be placed behind the sensitive plate and then a thick sheet of 

 glass to give a good even pressure. 



If, however, the negative has been stripped from the glass and is in the 

 form of a thin skin, the most perfect contact will be produced by trans- 

 ferring the negative on to the surface of the printing film, in such a manner 

 that it may be removed again after the exposure. 



This operation presents some difficulties, but I have found the following 

 method answer well. The sensitised and dried gelatine surface of the prin- 

 ting plate is covered with a very thin even coating of wax dissolved in 

 turpentine or benzole. The plate is then placed in a dish containing suffi- 

 cient spirits of wine to cover it. The thin negative film is laid down upon 

 the gelatine in its proper position, the plate and film are then removed from 

 the spirit, and the negative film carefully squeegeed into close contact with 

 the gelatine surface. The plate is then covered with a few thicknesses of 

 blotting paper, under a thick glass plate, and allowed to dry. When dry, the 

 plate is ready for exposure. After exposure, the negative film is removed 

 from the gelatine surface ; and, if sufficient wax was used and the film is 

 fairly tough, it comes away without tearing. Should it tear, it should 

 at once be dissolved off with ether, or there will be a continuating action of 

 light on the parts of the gelatine surface protected by the negative film, so 

 that they will print darker than the rest of the plate. Before printing, the 

 wax should be removed from the gelatine with turpentine. The object of 

 effecting the transfer in a bath of spirits of wine is, that neither the 

 gelatine, wax, bichromate of potash or negative film are in any way affected 

 by it. 



The duration of the exposure to light varies from 10 minutes in the 

 sun for a clear line subject, to from 25 to 50 minutes for a subject in half 

 tones, according to the density of the negative and the intensity of the light. 

 It is almost impossible to judge of the progress of the printing by in- 

 spection, and it is necessary to use an actinometer as a guide to the exj^osure. 

 The following form of actinometer has been found to answer well for the 

 purpose. It consists principally of a box, in the lid of which is fixed a 

 translucent scale divided in 14 squares of different densities. No. 1 being 



