414 Dynamic Theory. 



green, and iodide of silver only by violet and indigo light. Various 

 judicious mixtures of these compounds of silver of course are variously 

 affected, and produce various artistic effects. But it is remarkable that 

 bromide of silver, which by itself is not affected by tones below green, 

 can be made sensitive to yellow, orange and red by mixing with it sub- 

 stances that absorb those colors. In this way it has become possible to 

 take photographs with light of these lower tones. 1 



The Fluorescent property of Sulphate of Quinine has been mentioned. 

 This property may be curiously taken advantage of to photograph invis- 

 ible pictures. The picture is to be drawn upon paper with a concen- 

 trated solution of the sulphate of quinine. To the unassisted eye such 

 picture is nearly invisible. But if photographed the picture "takes " 

 distinctly black or dark. The reason is, that the rays which affect the 

 chemicals on the photographic plate are the indigo, violet and ultra- 

 violet, and that these rays when reflected from the sulphate of quinine 

 are reduced in wave length and become perhaps green rays, which are 

 not able to affect the chemicals on the plate. The effect then is just 

 the same as if the drawing were black, which absorbs these rays ( as 

 well as all others ). It is obvious that any kind of light that has the 

 force to shake apart the chemical combination of the minerals on the 

 plate, will take photographs. Such conditions are found in light from 

 many different sources. The magnesium light, made by burning a 

 Magnesium wire; the Drummond light, made by the flame from the oxy- 

 hydrogen blow pipe blown against a piece of lime ; the electric light, 

 and light from an incandescent piece of iron, &c. , will do to take pho- 

 tographs with, because they emit light of all wave lengths, including, 

 of course, the violet and ultra-violet, so much concerned in this busi- 

 ness. But those lights emitting an incomplete scale of tones, will not 

 do unless the tones are those at the top of the scale. The white Ben- 

 gal light of arsenic, the flames of the blue Bengal light and those of 

 burning sulphur possess the requisite tones. But the tones of the com- 

 mon oil lamps, kerosene, and coal gas are too low, belonging to the 

 red, yellow and orange part of the scale, chiefly. Yet, as before men- 

 tioned, these are the rays that effect the chief changes required in the 

 growth of plants, and plants will keep awake and grow in such light. 

 Light and heat being merely different periods of vibration of the same 

 ethereal substance, there is no difference in principle between the actions 

 of the two. When heat is absorbed, it is because waves of a certain 

 period of vibration have found a place where their vibrations can be 

 piled on top of each other in the production of heat or work in a body 

 that is, they become sensible or latent heat in a body. It is the same 

 with light when it is absorbed it may simply add to the heat of the 



1 A discovery of Vogel. 



