1878.] to the Reproduction of Maps and Plans. 95 



almost transparent, while No. 11< is almost quite ojDaque ; and numbers cor- 

 responding to the densities are painted in opaque colour on the scale. The 

 scale is made by taking a collodion negative of a drawing shaded in tints 

 of different strengths, and should be intensified so as to correspond in 

 density with the kind of negatives it is intended to be used with. 



The body of the box contains a block for carrying the sensitive surface, 

 which may be spread on paper or on a glass plate, and a strip of vulcanised 

 rubber below it presses the block into close contact with the scale. 



I prefer to use in the actinometer a sensitive film of the same com- 

 position as the printing plate ; small slips of glass are therefore coated with 

 the gelatine mixture, sensitised, dried and exposed to light at the same 

 time and in the same manner as the printing plates, and thus the progress of 

 the action of light can be w^atched and timed very closely. 



When the exposure to light is considered sufficient, the printing plate 

 is removed from the pressure-frame and laid, gelatine side downwards, on a 

 board covered with black cloth. The back, or under surface, of the gelatine 

 is then exposed to light, for about 10 minutes, to thoroughly harden the 

 gelatine and prevent it from swelling too much in the after processes. It 

 is well to conduct this second exposure under a piece of ground glass, in 

 order to prevent any scratches that may be on the back of the glass from 

 showing as white lines in the print. The edges of the j)late are then pro- 

 tected by strips of paper coated with solution of india-rubber, and Avhen the 

 india-rubber is dry, the plate is soaked in water until all the soluble bichro- 

 mate has been removed, and is then ready for printing. 



The plates can be printed in a lithograj)hic press, but then they require to 

 be fixed on a level stone with plaster of Paris. It has been found, however, 

 more convenient, and in some respects better, to print them with vertical 

 pressure in an ordinary Albion platen press ; and in order to prevent the glass 

 being broken, the bed of the press is fitted with two or three thicknesses of 

 kamptulicon, besides a sheet of vulcanised india-rubber on which the plate 

 rests. It is also desirable to place a piece of white paper over the bedding 

 in order to enable the state of the plate when it is being inked up, to be 

 better seen. 



The inking in requires great skill and care on the part of the printer 

 and is the most difficult part of the whole operation. The plate having 

 been well soaked in water is laid on the press, and after being wiped to 

 remove the excess of moisture, is inked in, if a line subject, with an ordinary 

 lithographic roller charged with an ink comj)osed of lithographic chalk ink 

 thinned with a little olive oil, followed by rolling with a smooth roller to 

 clear away the superfluous ink ; a mask of the required size is laid on the 

 plate to preserve the margins clean ; over this comes the printing paper 

 covered with a piece of soft felt, to drive the paper well into the hollows of 



