134 THE BOOK OF THE LANTERN. Melt by heat, but not above 120° Fahr. Then in a yellow light, pour B into A, stirring rapidly all the time, and finally adding C. Allow the emulsion thus made to remain for one hour, at the temperature already stated, and then put aside in a dish to set. The washing, filtering, and coating operations are the same as those described for bromide plate making. The bright yellow light allowable is a great help to comfortable working of this process. Chloride plates are useless, on account of their slowness, where a slide has to be reduced, by means of the camera, from a negative larger than itself. Nor do I advise the amateur to adopt them unless he can work by daylight, or is fortunately situated like one I know, who lives oppo¬ site to an enterprising tailor who displays an electric arc light in front of his door. For the chloride plate is most insensitive to yellow light, such as that afforded by gas. For this reason, most commercial makers advise that the light chosen should be that procured by burning an inch of magnesium wire at a distance of so many inches from the printing-frame. This advice is not difficult to follow, but it is very difficult to make two pieces of wire give out exactly the same amount of light ; for magnesium wire has a habit of dropping down in a languid manner under the influence of its own heat, and going out suddenly when it ought to shed its radiance abroad. With diffused daylight all is plain sailing. The negative, with its chloride plate in contact with it, is exposed, say, for three seconds to daylight, and is then dropped into the developer. Here is a good one, devised, if I remember rightly, by Mr. Edwards :—