1 66 THE ART OF PROJECTING. 



appearing more like colored spots upon the screen, 

 than characteristic lines. Very small fragments of 

 metals or. other conducting substances may be used in 

 this way. It is not, however, to be understood that 

 such spectra can be made large like those produced 

 by the oxyhydrogen or electric light. They may, how- 

 ever, be shown as spectra a foot long and the lines two 

 or three inches long, and thus be useful in places 

 where the more pretentious ways of projecting spectra 

 are not to be had. 



If an induction coil capable of giving a spark two or 

 three inches long is not to be had, a common electrical 

 machine will answer ; for the elements to be employed 

 may be fastened into retort stands, and separated an 

 inch or two as if simply to pass sparks from one to the 

 other, these connected by wires to the Holtz or other 

 similar electrical machine. The sparks may be pro- 

 jected as in the case with induction coils. 



TO PROJECT AN ELECTRIC SPARK, 



Suppose, from a Holtz machine. So place the 

 machine that the spark between the terminals shall be 

 parallel to the screen to receive the image. Take a 

 lens with a foot focus and with as large a diameter as 

 possible, say four or five inches, and mounted in a 

 broad frame (Fig. 15). If this be placed at the proper 

 distance and height from the terminals of the electric 

 machine, a spark will be projected on the screen much 

 magnified, all its zigzag lines amplified. There is no 

 difficulty in making an ordinary three or four inch 

 spark appear to be six or eight feet long if the screen 

 be fifteen or twenty feet away. It will be necessary to 

 have the room quite dark, and also, to have the screen 

 shielded from the light of the spark, but the frame of 



