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On a New Anti-vibration Turntray. 



By W. K. Bridgman, L.D.S. 



(Read September 22nd, 1876.) 



Plate XIX. (Upper Part). 



The immense superiority in the performance of an object-glass 

 under the entire absence of tremor is such as to render its practical 

 attainment a very great desideratum. That this was fully recog- 

 nised by the late Andrew Boss is sufficiently evidenced by his 

 endeavouring to obtain immunity from it by supporting the feet of 

 his instrument upon a combination of felt pads and springs, which, 

 however, was abandoned as insufficient, simply because it was incorrect 

 in principle. It is a law in nature, that working power is not derived 

 from any one force alone, but it is the swn of the difference between 

 tiro opposing forces, the preponderance of either constituting the 

 amount of the potentiality in store. Hence, if a board be supported 

 upon four bent springs, one at each corner, as shown in section at 

 Fig. 1, these, when weighted, will tend to become straightened, and 

 thus shift their points of contact with the support, the friction of 

 which would cause vibration, which would be communicated to the 

 board ; but if these four springs be made to rest upon four other 

 similar springs in an inverted position — as in Fig. 2 — so as to oppose 

 two forces in contrary directions, all vibration imparted to the lower 

 springs will be taken up and neutralised or destroyed by the upper 

 ones, and leave the top board comparatively at rest. 



Now, as vulcanised india-rubber is often an efficient substitute for 

 steel springs, it has been had recourse to in the present instance 

 with a fair amount of success. Two boards about three-eighths of an 

 inch thick, and sixteen inches long by ten inches wide, are notcl ed 

 at the corners — as shown in Fig. 3, a a — so as to admit of a small 

 sewing-machine thick india-rubber ring being stretched over them, 

 as seen in Fig. 4. The boards are then stretched apart, and four 

 other rings are compressed in edgeways"," as at b. There will then be 

 the tension of the one set of rings opposed to the expansion of the 



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