98 Li.-Col. Wat crhousc exhibited photographs. [March, 



the map before you, the drawing took about 6 months and the reproduc 

 tion has taken about 5 weeks. Had the map been engraved the drawing 

 would not have taken so long but the engraving would probably have 

 taken a year to complete. 



Dr. Burgess' two drawings were crucial tests of the process. They 

 were very finely drawn and reduced very considerably. The deli- 

 cacy and perfection of the results leave very little to be desired, as you 

 will see from the prints, and no other photographic process I am acquaint- 

 ed with would have rendered such close fine work so perfectly. 



Another process of heliogravure — known as photo-etching — employed 

 jn the office, is even quicker and more economical than the photo-electro- 

 type but not quite so certain. Some specimens of it are on the table. 

 This process is the exact opposite of the former, a direct nega- 

 tive is used, but a transparency has to be taken from it : this we prefer 

 to do by the autotype process, intensifying the gelatine image with per- 

 manganate of potash. From this transparency a negative autotype print 

 is made and developed upon a highly polished copper plate prepared for 

 engraving. Before the copper plate receives the gelatine image a resin- 

 ous grain of powdered bitumen is applied to it, which not only gives a 

 firm hold to the gelatine film during the biting, but breaks up the image 

 into a series of fine points which preserve the proper gradation of the 

 half tints and enable the different parts of the engraved image to hold 

 the ink in proper proportion. The resinous grain is fixed to the plate 

 by subjecting it for a few seconds or a minute to the fumes of benzole. 



The margins and back of the plate being protected with varnish, 

 the copper plate with the negative gelatine image is immersed in an 

 almost saturated solution of perchloride of iron which hardens the 

 gelatine, though slowly permeating it, and attacks the copper, so that 

 first the bare parts in the deep shadows of the picture, where there is 

 little or no gelatine, are bitten ; then the next darker tints, and so on 

 until the high lights are just on the point of being attacked. With 

 some subjects a single bath of perchloride is sufficient — with others 

 it is advisable to begin with a very strong solution of the perchloride and 

 pass the plate through a series of baths of decreasing strength. It is 

 marvellous with what delicacy and perfection the most delicate grada- 

 tions of half tint, even in the lightest shades, are reproduced on these 

 plates. The biting takes only a few minutes and the whole operation 

 of preparing a plate can be finished in a day. It is therefore exceedingly 

 quick and the cost of materials is very small. The method is suitable 

 either for line or half-tone subjects, but is perhaps most successful with 

 the latter. 



