SOUND 



269 



horizontally in one of the sockets of a pillar-stand, so as to be 

 adjusted at the height of the optic axis of the lantern. On 

 the large open end, fit any of several short brass tubes, over 

 which are strained membranes of very thin rubber, held by 

 elastic bands catching in grooves cut round the ends of 

 these short tubes. On these membranes mirrors are cemented ; 

 and by stretching the rubber a little one side or the other, or 

 by changing one of these caps for another, a good position is 

 sure to be readily obtained. This is a very simple, easy, and 

 elegant experiment. 



147. Manometric Flames. Another method is by Konig's 

 manometric flames, which are projected easily, and better seen 

 in some respects than by direct 

 vision. They can be projected even 

 by the simple apparatus usually sold, 

 with capsule and revolving mirror 

 upon one base-board ; in which case 

 the focussing lens must be of long 

 focus, and stand between the re- 

 volving mirror and the screen. The 

 lantern itself has no place in this 

 experiment, the flame only being 

 focussed ; and it is necessary that 

 this be enclosed in an opaque 

 chimney of some kind, only open 

 towards the mirror, in order that 

 the screen and room may be as 

 dark as possible. I have got fair 

 results this way ; and if more bril- 

 liance be required, it is easily ob- 

 tained by ' enriching ' the gas, 



passing it through benzoline, or over naphthalene heated by 

 a Bunsen burner in the albo- carbon manner. The flame will 

 be very greatly brightened by this expedient. 



But it is very much better to have the capsule and jet 



FIG. 141 



