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196 THE ADVANCE OF PHOTOGRAPHY
that employed in the manufacture of plates, but far less rapid. The paper, however, must be carefully preserved from the action of white light, otherwise on developing, a dirty greyish appearance will result, and no good whites will appear in the print. Such a result will also occur if the paper has been greatly over-exposed under the negative, or it may take place, as also happens in the case of the plate, by employing a too strong developer.
Many varieties of bromide and gaslight papers can now be purchased ; and since full instructions are issued by the makers as to the exposure, development and subsequent manipulations so as to avoid fogs, stains and other imperfections, no more will be here said, than to mention the fact, that in order to obtain good results from this class of prepared paper great care must be taken and every precaution adopted to ensure absolute cleanliness in the baths used.
The negatives which give best results with bromide papers are comparatively thin plates, full of detail, and it is necessary that the plates should be developed and fixed in such a manner as to avoid stains.
It is possible to tone bromide prints so as to obtain good red, green, and blue colours, as well as such intermediate tints as brown and sepia.
Platinotype. — Lastly, a brief mention must be made of Platinotype paper, which is used for obtaining prints, perhaps the most permanent form of all, since in them the image consists of metallic platinum, and platinum does not oxidize, and is practically unaffected by sulphur and chlorine. This paper was invented by Willis in 1873, improved in 1878, and again in 1880. At one time the platinum was added to the paper in the process of development, but it is now upon the prepared paper itself. The paper must be kept absolutely dry.
There are several modifications of this paper, but as a rule it is sensitized with a mixture of ferric oxalate and a