PERFORATING GLASS.—To make a hole in glass, an ordinary well-tempered steel drill is the best tool. The drill must be kept moistened with turpentine in which some camphor has been dissolved, or with diluted sulphuric acid, or with ordinary kerosene oil.
PERrIANENCY.—The question of the permanency of photographic images is one that has been much discussed. It is a well-known fact that silver prints made thirty or forty years ago have almost entirely faded away. Here and there prints will be found of that age in quite good condition, the reason lying, in all probability, in the method of manufacture adopted. It is pretty generally known that hypo remaining in the picture will soon cause it to fade. To thoroughly eliminate this, several so-called hypo eliminators have been recommended. These should never be used, for while removing the hypo from the film they leave another composition which is more likely to cause the print to fade than the hypo salt itself. Good washing is the only true eliminator. This must be thorough and not too long. Too prolonged washing tends to decompose the prints, and will cause them to fade more rapidly. The larger the quantity of gold used in the toning
process, the longer will be the lasting qualities of the prints. This should be borne in mind by those economical workers who endeavor to make a tube of gold tone a large number of sheets. Another cause of fading is due to impurities in the mount and the mountant. Unmounted prints placed between sheets of chemically pure paper will last considerably longer than mourtted ones. Good quality mounts and a pure, simple paste should be used.
Silver prints are probably the least permanent of any photographic pictures. Gelatino chloride of silver papers (Celerotype, Aristotype, etc.), are much more permanent than albumin ized paper.
Bromide and platinotype prints are also considered as permanent.
The most permanent are undoubtedly the pictures produced by the carbon or Woodbury process. In these the images are composed simply of gelatine and lampblack, two permanent substances.