1878.] 1o the Reproilucfion of Maps and Plans. Hi) 



every purpose. Tlie ' cyanotjpe' process, before described, might also be 

 used, but the blue is much more intense and would be liable to produce 

 a double image on the copy. 



Besides its uses in the Survey Department, the blue-print method is 

 also appreciated and utilised by engineers and other public officers in India, 

 who desire from time to time to show alterations or improvements on a 

 standard plan, or wish to make use of a standard map for showing their 

 own special requirements. 



Bicliromnie-prints. — In the case of miscellaneous subjects whicli it 

 is desirable to lithograph, photography can also be usefully applied in giving 

 the lithographic draughtsman an accurate tracing over which he may make 

 his transfer-drawing in the usual way. This was formerly done by making 

 a silver-print on the required scale, and then either tracing over it on 

 lithographic tracing-transfer paper, or coating the print itself with the 

 composition used for lithographic transfer paper and re-drawing on the 

 print itself ; but in either case, the dark colour of the photograph inter- 

 feres very much with the drawing. 



A method of overcoming this difficulty has been suggested by Mr. 

 Eraser S. Crawford of the Government Photo-lithographic Office, Adelaide, 

 S. Australia, and has proved exceedingly useful here in Calcutta. A print 

 is taken from the negative, on the paper prepared with gelatine and bichro- 

 mate of potash for the photozincographic transfers, but it must be printed 

 as deeply as possible, so that the lines may remain clearly visible after the 

 bichromate has been washed out. Instead of inking the print, it is simply 

 washed till all the soluble bichromate is removed, and is then dried. The 

 surface of the print is coated with the ordinary composition of starch or 

 isinglass used for preparing lithographic transfer paper and, according as 

 the drawing is to be executed with the pen or chalk, receives a smooth or 

 grained surface by passing it through the press either on a polished or grained 

 copper plate. The draughtsman then makes his drawing with autographic 

 ink or lithographic chalk over the faint russet image on the photographic 

 print. An ordinary silver-print can be given as a guide in cases where the 

 bichromate print is not sufficiently distinct for the details to be easily made 

 out. This method is very suitable for copying maps, sketches, or photo- 

 graphs from nature, especially if the former are to be on a smaller scale 

 than the original. The light colour of the photographic print renders it 

 easy to see the effect of the drawing above it. The saving of time and 

 labour in tracing and the superior accuracy of the ground-work are also 

 great advantages gained by its use. 



Flioto graphing on Stone. — The following method of photographing 

 direct on stone may sometimes be of use with the same object, when the 

 lithographic drawing is to be made on the stone itself and not transferred. 



