400 THE POPULAR SCIENCE MONTHLY. 



tests of chloride of silver, among chemists, is that it shall darken in 

 the light. Here I have a little bulb of it which was prepared, dried 

 carefully, and sealed up. It has been exposed for months to the light, 

 and is as pure a white as it was the first day it was put into the bulb. 

 Another experiment was made at the same time ; but, unfortunately, 

 as I thought then, a small globule of mercury got into the vacuum, and 

 was sealed up with the chloride ; the consequence was that the chlo- 

 ride of silver immediately darkened : although the mercury was not in 

 contact with the salt, the chlorine flew to the mercury, and formed 

 chloride of mercury. This is an instructive experiment, showing that 

 chloride of silver will darken merely in the presence of something that 

 will mop up the chlorine. Silver iodide, when exposed to light, splits 

 up into silver subiodide and iodine, and silver bromide into silver sub- 

 bromide and bromine. Now, in order that there shall be a ready dark- 

 ening of either of these salts, you must have something which will 

 absorb the iodine or bromine (or, in the case of the latter, allow it to 

 escape), according to the salt you expose to the light. This some- 

 thing is the sensitizer. 



One point that has exercised the minds of a great many photog- 

 raphers is the illumination of their dark rooms. [The lecturer having 

 shown the relation of the several parts of the solar spectrum with the 

 absorption properties of different substances used in photography, pro- 

 ceeded to demonstrate the effect of differently colored glasses upon 

 the passage of rays, and announced his conclusions.] If photogra- 

 phers want to have an absolutely safe light in developing their pict- 

 ures, let them glaze their studios withe obalt glass and stained red, and 

 they will get nothing but the light of that particular refrangibility, 

 which will not affect any gelatine plate of the ordinary type. You 

 may glaze and glaze with ruby, but you will never get rid of blue 

 light entirely. Of course, it diminishes with every thickness you take. 

 If you want to use ordinary plates, which are not so sensitive that 

 you can not look at them, my advice is to use a combination of stained 

 red and ruby glass, which will give you a comfortable light to work 

 in, for it cuts off the blue and leaves the red in a brilliant patch. If 

 the operator wishes to be still more secure, let him use a combination 

 of cobalt glass and stained-red glass. A combination of red and 

 green is a fairly safe light for iodide plates or ordinary plates, but not 

 for gelatine plates, which are very sensitive. Next we come to a series 

 of pretty colors, which may be very useful to us : magenta, with which 

 the yellow is cut out entirely, and the green, leaving the blue, violet, 

 and orange ; aurine and chrysoidine, which cut off the blue ; a com- 

 bination of magenta and aurine, which gives a perfect red light, and 

 is very good indeed for the photographic studio ; and scarlet and 

 aurine, which give the same effect. If all means of securing the 

 right light fail, the photographer may use the ferrous oxalate de- 

 veloper, for you may bring the most sensitive plate out into a white 



