118 J. Waterhouse — The Application of FliotograpTiy [No. 2, 



use of the draughtsman, engraver or lithographer, thus saving all the labour 

 of hand-tracing and obtaining also a far more accurate image than could 

 possibly be obtained in any other way. 



Blue- Prints. — It is well known that when photographed a pale blue 

 colour acts exactly as if it were white. If, therefore, we can obtain by 

 photography an accurate image of any original drawing or other subject in 

 pale blue ink, either on the same, an enlarged or reduced scale, it will be 

 possible to redraw the whole or part with black ink over the blue print, in a 

 style suitable to be again reproduced by photography, without fear of obtain- 

 ing a double image. It will also be obvious that the blue ground-work will 

 be more complete and more accurate than any tracing by hand or pantograph 

 could i^ossibly be. 



It has already been stated that this method has been extensively used 

 in the Survey of India for making reductions of maps to smaller scales, and 

 at the same time generalising the details on the large-scale maps so as to 

 adapt them to and render them more suitable for the smaller scale. 



The same system may be applied to the production of maps or plans 

 in several colours, thus : supposing a map is to be reduced and printed in 

 three colours, black for the outline and names, brown for the hills, and blue 

 for the streams and other water. 



Three blue prints are given to the draughtsman who draws on the first 

 only the outline and names, on the second the hills, and on the third the 

 water. These three drawings are then very carefully photographed all 

 on the same scale, and transfers are made from them on to three stones or 

 zinc plates, which are then used for printing the different colours of the map 

 just as in ordinary chromo-lithography. 



There is no limit to the number of colours that may be employed, and 

 as the blue-prints are all on the same scale, very perfect ' register' may be 

 secured, if due care be taken in the drawing and subsequent photograjDhic 

 operations. 



The advantage of using photo-zincography for preparing the blue- 

 prints is that in reducing a large-scale map, the transfers of the several 

 sections may be joined together and printed off in one sheet, and thus 

 may be redrawn in a more complete manner than if the sections had each to 

 be drawn separately and afterwards joined up. 



It is not, however, always desirable or possible to obtain the blue 

 prints by photozincography, and they may be obtained in a more direct 

 manner by coating paper with bichromate of potash and gelatine, exposing 

 under a negative, well washing the print with hot water to dissolve the 

 gelatine, then steeping it in a solution of proto-sulphate of iron, again well 

 washing and then applying a solution of ferrocyanide of potassium, and 

 finally well washing. A pale blue print is thus obtained which will answer 



