112 C. Bar us — Displacement l?ite/fero?neter. 



inclined to the horizontal, either by the hinge indicated in the 

 figure, or by the special device shown in fig. 8, where the tele- 

 scope or collimator reposes on Y's made of strips of elastic 

 brass aa. These are so adjusted that the end of T at a r is 

 naturally higher than at a. The ring b and the thumb screw c 

 then lowers this end against the upward pressure of all the 

 springs. Reading telescopes so mounted are firm and the 

 device is very convenient if a slight inclination is to be 

 imparted. They are removed by loosening c, and slipping T 

 out of the ring, b. 



The tube a ends on the left in the cross-coupling, which 

 also admits the adjustable standard a" and affords an attach- 

 ment for the braces bb (U-shaped in section), the other ends 

 of which are bolted down to the nearer feet of the tripod. 

 Thus A is held sufficiently rigid by the braced system aa" bb. 

 An inch or a 3/4 inch objective and a 6 inch focus is sufficient 

 and by reason of its lightness perhaps preferable to a larger 

 and heavier tube. The slit may usually be opened about l/2 rani . 



In a similar way steadiness, elevation and inclination of the 

 telescope E is secured, the tube e and e" (adjustable foot) and 

 the braces b" terminating in the cross-coupling e' as has been 

 suggested. An inch objective and a 6 inch focus is adequate. 

 Cross hairs are convenient but not necessary, as the spectrum 

 lines are available when sunlight is used. If the arc light is 

 used, strong sodium lines are usually in the field with the 

 spectrum. 



The opaque mirror M is controlled by three leveling screws 

 (horizontal and vertical axes) and a suitable spring in the cap- 

 sule D. It is adjusted vertically like the telescope and kept 

 firm by the tubes d and d" (adjustable foot) and braces V b', all 

 parts meeting at the cross-coupling df. The braces b' b' are of 

 equal length. Hence they may be bolted down to two of the 

 feet of the tripod in succession, while the tube d together 

 with the plate iTtake the three positions at 30°, 90°, and 150° 

 to the rod a. The grating G- does not turn with K but must 

 be specially adjusted to corresponding angles of 15°, 45°, and 

 75°, as easily determined by the reflected rays. 



Finally the slide micrometer is sustained by the tubes c and 

 c" (adjustable foot) and the braces bb, all parts meeting in the 

 cross-coupling c'. The latter carries the table Z, to which the 

 slide micrometer C, with its drum at F, is bolted down. N is 

 the opaque mirror adjusted by three leveling screws and a 

 spring (horizontal and vertical axes) within the capsule B. The 

 slide should have from 1 inch to 2 inches of clear play and its 

 displacements should be determinable to about , 00005 cm . The 

 opaque mirrors M and N may both be silvered on the back 

 and thus last indefinitely. 



