160 EARL DE GREY'S ADDRESS. [May 28, 1860. 



Murchison, Director of the Geological Survey, has published them, 

 with the geological structure of the country represented on them. 

 Captain Washington, r.n., Hydrographer to the Admiralty, is also 

 supplied with the copies of the plans, and with distances and 

 heights, to enable him to connect his hydrographical charts with 

 the Ordnance Survey, and thus the topography, hydrography, and 

 geology, have one uniform accurate basis. 



I have referred to the advantage to the survey which the 

 introduction of photography by Sir H. James has produced ; it has 

 enabled him to do that which would otherwise have been impos- 

 sible, that is the production of the series of maps required in any 

 time which could be possibly allowed for the work. And in this 

 last Eeport Sir H. James has given an account of a method now 

 employed for the reduction and transfer of the maps to copper, 

 zinc, or stone, which is not only applicable to the immediate pur- 

 poses of the survey, but which will be found of inestimable advan- 

 tage for the production and printing of fac-similes of any printed 

 or manuscript document, or outline engraving. This * discovery is 

 so important, that I think it will gratif}^ the Fellows of the Society 

 if I give a concise account of it. 



The fact that a solution of the bichromate of potash becomes 

 insoluble under the action of light is the basis of the operation ; 

 and to render this available for the purpose of printing on zinc or 

 stone a highly-intensified negative photograph is first taken with 

 collodion on glass ; a sheet of thin tracing paper is then coated with 

 a saturated solution of the bichromate of potash mixed with gum- 

 water; when dried, this paper is exposed in the printing-frame, 

 under the negative, for two or three minutes in the light. The 

 action of the light through the lines or writing makes that part of the 

 composition insoluble, while the remainder remains soluble, and can 

 be removed. To effect this the bichromate positive is laid on a sheet 

 of zinc, previously charged with lithographic ink, and passed three 

 or four times through a printing-press. On taking the paper from 

 the plate the entire surface is uniformly covered with ink ; but on 

 submerging the paper in a shallow vessel of hot water with a little 

 gum in it, and gently brushing over the surface with a flat camel- 

 hair brush, all the soluble portion of the composition, with the ink 

 attached to it, is removed, and the outline of the MS. or print is 

 produced quite perfect, and charged with ink, and, when dried, it 

 is at once ready for transfer to zinc or stone or the waxed surface 

 of a copper plate. Sir H. James has called this art Photo-zinco- 



