316 OPTICAL PROJECTION 



behind the arc-light to throw out a parallel beam. If from the 

 aperture a tube of the same diameter, silvered and polished on 

 the inside, extends for two or three feet, the heat is kept from 

 scattering. (Condensing lenses are unsuitable for this experi- 

 ment, as glass is a powerful absorbent of invisible heat rays.) 

 A thin globular flask with a neck, about 4 inches diameter, is 

 filled with the solution and held in the parallel beam ; at its 

 focus black paper and other substances will be ignited. 



This method is not very suitable for the oxy-hydrogen 

 light, because the lime does not radiate much heat to the 

 back, where the mirror must be placed. The best method 

 is to hold or adjust the globular flask, of the thinnest glass, 

 five or six inches from the naked lime. It is all the better if 

 a reflecting tube occupy the space between, as it will concen- 

 trate the heat considerably ; and the densest and hardest 

 limes must be used, these being much superior in radiating 

 power. A powerful jet of -j^-inch bore will make the whole 

 lime incandescent, and give enormous heat, which will be 

 quite sufficient at the conjugate focus of the flask. So near 

 the lime, however, I strongly advise the employment of carbon 

 tetra-chloride as the solvent. It does not give an absolutely 

 opaque solution like carbon disulphide, a little violet struggling 

 through ; but this is hardly perceptible, and the solution is 

 not dangerous, as disulphide so close to the lime most 

 certainly is. This will be found a very inexpensive, simple, 

 and effective arrangement. 



The energy of nearly invisible actinic rays may be shown 

 either by taking a photograph through several blue glasses ; 

 or by exploding (with the usual precautions) at a rather long 

 conjugate focus of the condensers, through similar nearly 

 dark glasses, thin glass bulbs filled with chlorine and 

 hydrogen gases, obtainable from the makers of physical 

 apparatus. Explosion will be prevented by red or yellow 

 glasses, but will at once take place through the blue, even 

 with a good lime -light ; but if there is any difficulty, burning 



