64 THE ART OF PROJECTING. 



the beam horizontally, the flame will appear drawn out 

 into a band of light, due to persistence of vision. But 

 if the flame be not a bright one, the image will be too 

 dim to be useful, if the screen is ten or fifteen feet 

 away. The intermittent character of the singing hy- 

 drogen flame can be shown in ' this way, but it is much 

 better to use common gas in place of hydrogen, as the 

 flame is much brighter. The flame of common gas 

 may be made still brighter by passing it through ben- 

 zole or naptha, or tow saturated with ether. The room 

 must be quite dark. (See Tyndall on Sounds p. 223.) 

 In the American edition of Atkinson's Ganot's Physics 

 is pictured Koenig's apparatus for observing manomet- 

 ric flames. In place of the rotating reflector use the 

 concave mirror, as above, and the same figures will ap- 

 pear upon the screen. 



One can make a tolerable substitute for that apparatus, 

 if g^as be not obtainable, by fastening over the mouth 

 of a small two-inch funnel, such as is used in chemical 

 laboratories, a piece of tissue-paper or thin rubber. A 

 piece of rubber tubing, two or three inches long, may 

 be drawn over the stem of the funnel, and the other 

 end drawn over the mouth of a common jeweler's blow- 

 pipe. A sheet of pasteboard may now be rolled so 

 large that the broad end of the funnel, which has the 

 tissue-paper pasted to it, may fit snugly in it. The 

 whole fixture may now be supported in any way, by 

 means of retort-stands. A gas- flame from a small 

 round orifice, or a common candle may be used for the 

 flame ; the end of the blow-pipe is to be inserted in the 

 blaze, with the opening upward. If now, either a com- 

 mon mirror be used to give angular motion to the re- 

 flected beam, or the concave mirror to reflect the flame 

 upon the screen, while a sound is made in the large 



