RECENT ADVANCES IN PHOTOGRAPHY. 401 



light, when developing in a dish with a covering of that substance 

 over it. 



In 1874 the discovery was made that an increased action of the 

 spectrum could be got by dyeing the film of sensitive collodion. If 

 you take one of the aniline dyes and expose it to the light behind a 

 piece of black paper, you get an image on the dye. What is the 

 meaning of tbat ? The meaning is, that the dye is oxidized, for, if you 

 apply an oxidizing agent, you get the same result. Dr. Vogel found 

 that if he dyed a plate with one of these fugitive dyes, he was able to 

 obtain an extension of the impressed spectrum, and he introduced the 

 term " optical sensitizer " to describe the fact. I object to the term, 

 for it gives a wrong impression of the action that takes place, which is 

 simply the reduction of the iodide or bromide of silver by the oxida- 

 tion of the dye, and the provision of a nucleus on which development 

 can take place. 



Collodion emulsions have been in vogue for seven or eight years, 

 although they have now been superseded, to a large extent, by gela- 

 tine emulsions. Whether the last be an improvement over the former 

 process or not, the collodion process is admirably adapted for land- 

 scape-work. If the emulsion is of silver bromide or chloride, it is 

 easily formed ; an iodide emulsion is more difficult. The point in 

 emulsion-making seems to be to get the precipitate in as fine particles 

 as possible, and it is said that this can only be obtained, except at very 

 great cost of time and trouble, by first adding the soluble bromide or 

 iodide to the collodion. If you take the trouble to add the silver to 

 the collodion first of all, the aspect of emulsion-making is entirely 

 changed, and you can get any amount of fineness by adding the iodide 

 or bromide to the silver contained in the collodion so long as you keep 

 the silver nitrate in excess. If you put the iodide into the collodion 

 first, and then add silver nitrate, you will find that you have precipi- 

 tated the iodide of silver at the bottom of the bottle, and in a form 

 which will not emulsify at all. My advice to those who wish to make 

 collodion or gelatine emulsion is, to add the silver to the collodion or 

 gelatine, and then add the haloid salts afterward, and you will get as 

 perfect an emulsion as you choose. 



It is a great comfort in the collodio-bromide process that the oper- 

 ator is able to give local intensity (a most desirable quality in all artis- 

 tic work) to the image. I do not believe any process is perfect until 

 that power is placed in the hands of the manipulator ; and the great 

 defect of the next. process to be mentioned is, that it does not give 

 that power, but leaves the operator at the mercy of his plate, on which 

 he must let come out what will. This next process is the gelatine 

 process, which may be described as one in which the silver bromide is 

 held in suspension in gelatine in the same way that in the previous 

 process it is held in collodion. Mr. Bennett showed how a gelatine 

 emulsion can be made very sensitive by keeping it at a comparatively 

 tol. xx:i. 26 



