Page:Popular Science Monthly Volume 22.djvu/418

404 can get an enlarged print. We may thus say that an advance has been made, when, by an ordinary magic-lantern, with a good negative, you can get a perfect enlarged print by development. Perhaps it will not have that luster which albumenized prints have, but it is a matter of taste whether you like that gloss or not.

As gelatine plates are now prepared they all have an excess of soluble bromide. While this is the case, the highest sensitiveness possible will not have been obtained. Dr. Eder has found that an increase of sensitiveness, two or three fold, may be produced by neutralizing this excess. The gelatine-plate makers have the problem to solve, how to get rid of any possible excess of soluble bromide in their films.

We will next consider what causes the destruction of the photographic image. You may destroy it by any substance which will readily part with oxygen. You can destroy it, for instance, by bichromate of potash, by any of the ferric salts, or by oxygen-yielding substances, like permanganate of potash, ozone, peroxide of hydrogen, or hydroxyl; in fact, there is hardly any substance which will part with oxygen, which will not destroy the developable image. The photographic image remains behind as a rule, though not always, but these re-agents prevent it becoming developable. Bromine also acts sometimes as a destructive agent, by escaping, when the exposure is too long, from the lower part of the bromide coating of the plate, and forming a fresh film of bromide at the surface after it has been acted on by the light.

A remarkable utilization of the oxidizing process has been proposed and carried out by M. Bolas. Wishing to reproduce an ordinary gelatine negative having the proper gradations of light and shade, he took a gelatine plate, immersed it in bichromate of potash, allowed the film to dry, and then exposed it to light behind the negative to be reproduced. In this exposure he had an oxidizing agent present in his film; the oxidized parts were acted upon by the light, leaving the other part intact; and by that means he got a reversed image. Oxidizing agents enable us also to get rid of fog. A gelatine plate, which has been fogged by exposure to light, can be cleared by immersing it in bichromate of potash.

I have learned in my experiments that halations, or the appearance of haloes around the picture can be prevented, by touching the back of the plate with asphaltum or some varnish; the reflection is toned down according to what medium is placed on the back of the plate. The most perfect cure for halation is Brunswick-black. It admits no reflection from the back of the plate, and thus enables the operator to get rid of every tendency to fuzziness of the image.

A most useful instrument has been introduced by Mr. Warnerke, which is known as a sensitometer, or measurer of sensitiveness, it consists of squares of colored gelatine of different opacities through