SOUND 



259 



in which the forks are inserted. The course of the incident 

 pencil oa between the mirrors s { and s 2 will be seen from the 

 diagram, any adjustment being easily made before tightening 

 the screws, by either turning the perpendicular fork a little 

 on its axis, or moving the horizontal one in its slot. Another 

 block and slot k is provided for using both forks perpen- 



FiG. 135. Mounting for Two Forks 



dicularly, the block k taking one out of the line of the other. 

 It will be seen that with this arrangement both forks can be 

 readily bowed by assistants, and will be firmly held. 



A very simple apparatus is shown in fig. 186, as made by 

 Miller, of Innsbruck, from a design by Prof. Pfaundler. Two 

 flat steel springs, P F, are mounted in a manner that needs no 

 further explanation than the diagram, one being adjustable in 

 position, and having a sliding weight. Both are furnished with 

 mirrors. The springs may be bowefl, or are readily set in motion 



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