chemicals. Exposure to light creates a change, but this remains latent until the film is immersed in a developing solution containing hydroquinine. This acts as a reducer on the exposed silver salts. Where light has fallen, the silver is reduced to its metallic form, often referred to as the grain. At this stage, films are still sensitive to light, so to stop any further development, the film is dipped in a slightly acid bath to change the pH, or simply washed. Then it is fixed in a solution of sodium thiosulphate. The sodium thiosulphate, usually known simply as ‘hypo’ dissolves away any of the silver that has not been reduced to its metallic state, so the exposed parts of the image remain as the negative, insensitive to light, and, after washing to remove the hypo, about as permanent as anything can be in this world. If you look at an ordinary negative closely, edge on, in oblique light, it is possible to see a faint impression of the image as a positive. This is because the silver image has depth, very slight, but enough to make it stand out in relief. In transferring the negative image to the metal plate, the photographic process was much the same, but an extremely high contrast developer was used, and instead of producing almost extremely shallow depth, the fixing and finishing was much more aggressive. The developed lines acted as a mask so that chemical etching could continue until the the lines in an illustration ended up standing out in high relief. Like the wood engravings, these metal blocks could be set up with the type for printing.
To make a block, the illustration was first drawn carefully in reverse onto the smooth face, and then the wood was etched away. Above is a rare survivor, a block drawn on, but never etched. With the emergence of this new technology, some of the original illustrations were copied, resized, and occasionally altered. In one case the original illustration of a limestone cave by the Lower Lake, Killarney, was not just resized, but the figure of a person was removed. In many of the memoirs towards the end of the series, the illustrations were produced from a Illustrations, such as these fossil plants, were drawn directly onto the stone for lithographic printing.
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mix of wood and metal blocks. Blocks, once made, could be re-used, and often were. Authors and publishers frequently borrowed blocks, but there was a limit to how many impressions could be made. Eventually blocks wore out, and when lines started to blur, it was time to discard or cut a replacement. Metal blocks started to make their appearance in the GSI collection in 1885, but they were not necessarily greeted as an improvement. The Director-General of the Geological Survey in 1892, Archibald Geikie, remarked distainfully that the photo-mechanical method was far removed from engraving on wood. As always, however, cost eventually won that argument.