L. Waldo—New Position Micrometer. 51 
Lindsay* this form of mounting the frame is most likely to 
secure permanency in the errors of the screw, so that when 
once determined they may be assumed to remain constant for 
long intervals. 
The micrometer frame /f/'/’ may be moved in the grooved 
bearings 64 ’b’ by a screw, which is not shown in the sketch, 
wig ome the micrometer. 
he eye pieces also have a parallel motion by rack and pinion. 
The screw head may be read by two pointers d and d’, and the 
whole revolutions are registered by a toothed wheel which gears 
into the thread cut on the micrometer head and shown at cc. 
The milled head, mm, is larger than the disc carrying the 
graduations. The bright web illumination is effected in a 
manner similar to that adopted by Alvan Clark & Sons. There 
is an aperture at ¢ protected by a glass cover and a similar one 
at a’ which allows the light from a lamp to impinge on the webs 
in the frame. 
makers, Messrs. Fauth & Co. of Washington, D. C. 
The following observations on the revolutions of the screw 
between 22°50 rev. and 27°50 rev. were made with the micro- 
scope comparator I have previously described. 
i Il. iit, Iv. 
Rey. d. d. mm. 
ad 5-07 +0°02 +0°00031 
23-000 4°95 —0°10 —0°00157 
5°03 —0°02 —0 
5°12 +0°07 +0°00110 
5°10 +0°05 +9°00079 
retin 5-00 —0°05 0-0 
50 5°03 —0°02 0°00031 
5°05 0°00 
5°07 +0°02 +0°00031 . 
eh 5-08 +.0°03 0 
5°16 +011 +0°00173 
5°03 —0°02 +0°00031 
25 
5°06 +0°01 +0°00016 
500 5°05 0°00 0°00000 
ee 5-03 —0-02 —0-00031 
27-500 5°07 + 0°02 +0°00031 
* Dun. Echt. Obs. Pub., vol. ii, p. 53. 
+ Proc. Am. Acad. ‘Arts and Sci. Bost., vol. xiii, for 1877-78, p. 352. 
