4 04 THE POPULAR SCIENCE MONTHLY. 



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 in- 

 crease of sensitiveness, two or three fold, may be produced by neutral- 

 izing 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 photo- 

 graphic image. You may destroy it by any substance which will 

 readily part with oxygen. You can destroy it, for instance, by bi- 

 chromate 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 pho- 

 tographic image remains behind as a rule, though not always, but these 

 re-agents prevent it becoming developable. Bromine also acts some- 

 times 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. Oxi- 

 dizing 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 f uzziness 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 



