68 



THE ART OF PROJECTING. 



hole bored through 

 it lengthwise, s o 

 that a smooth stem 

 may freely turn in 

 it. This stem 

 must have a large 

 head upon it, thro' 



which is bored a 



^^^' ^^' hole to receive the 



stem of the fork. Set-screws should be provided, to 

 fasten the stems in their proper places. These sup- 

 ports might be made of cast-iron, in which case they 

 would not need to be nearly so large. 



Next make four slides of iron, an inch and a half or 



two inches long, and bent so as to 

 slide upon the fork and be fixed with 

 a set -screw where it is wanted. 

 These are for loading the forks and 

 making them vibrate slower, as they 

 are nearer the ends. 



Lastly, each fork will need a small 

 mirror fastened to its end. The 

 Fig, 44. small, round pocket mirrors, about 



an inch in diameter, I have found to answer well ; but 

 care should be taken, in selecting these glasses, to get 

 Jf/ain mirrors. Most of these small ones are on poor 

 glass, and will spread a beam of light over a large 

 space. These mirrors may be fastened to the end of 

 the fork with the cement known as marine glue, and 

 will adhere strongly enough for all careful work ; but 

 sometimes these are fitted with a screw in the back, and 

 screwed into a tapped hole in the end of the fork. 



A still better way to fasten this small mirror, is to 

 cement to its back a piece of rubber as long as the 



