Page:Popular Science Monthly Volume 22.djvu/415

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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 that? 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 oxidation 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Collodion emulsions have been in vogue for seven or eight years, although they have now been superseded, to a large extent, by gelatine emulsions. Whether the last be an improvement over the former process or not, the collodion process is admirably adapted for landscape-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 precipitated 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 operator is able to give local intensity (a most desirable quality in all artistic 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