186 LETTERS ON NATURAL MAGIC. 



illumination of the apartment with these lights in succes- 

 sion would add to the variety and wonder of the exhibition. 

 The red light might perhaps be procured in sufficient 

 quantity from the nitrate and other salts of strontian ; 

 but it would be difficult to obtain a blue flame of sufficient 

 intensity for the suitable illumination of a large room. 

 Brilliant white lights, however, might be used, having for 

 screens glass troughs containing a mass one or two inches 

 thick of a solution of the ammoniacal carbonate of copper. 

 This solution absorbs all the rays of the spectrum but the 

 blue, and the intensity of the blue light thus produced 

 would increase in the same proportion as the white light 

 employed. 



Among the numerous experiments with which science 

 astonishes ' and sometimes even strikes terror into the 

 ignorant, there is none more calculated to produce this 

 effect than that of displaying to the eye in absolute dark- 

 ness the legend or inscription upon a coin. To do this, 

 take a silver coin (I have always used an old one), and 

 after polishing the surface as much as possible, make the 

 parts of it which are raised rough by the action of an acid, 

 the parts not raised, or those which are to be rendered 

 darkest, retaining their polish. If the coin thus prepared 

 is placed upon a mass of red-hot iron, and removed into a 

 dark room, the inscription upon it will become less 

 luminous than the rest, so that it may be distinctly read 

 by the spectator. The mass of red-hot iron should be 

 concealed from the observer's eye, both for the purpose of 

 rendering the eye fitter for observing the effect, and of 

 removing all doubt that the inscription is really read in 

 the dark, that is, without receiving any light, direct or 

 reflected, from any other body. If, in place of polishing 

 the depressed parts, and roughening its raised parts, we 

 make the raised parts polished, and roughen the depressed 

 parts, the inscription will now be less luminous than the 



