268 



C. Barus — Simple Screw Micrometer. 



reappeared with each complete turn, after the Fraunhofer 

 micrometer had also been correspondingly displaced. Owing 

 to the length (three inches) of the new screw used, it was nec- 

 essary to use the Fraunhofer screw four times in succession, by 

 introducing thick glass compensators into the ray coming from 

 the new screw. These were made with thick plates of glass 

 and added at the places indicated by a, a.\ a", iu the figure. 

 The total thicknesses of the piles of plates were 



•on 

 •oro 



•072 

 •071 

 •070 

 •069 



tWv>U 



d 



&UI& 



V 



a 



M 



a!' 



8 16 



& 40 



56 6f 71 



96 104- 



at a, 2-10 cm., 3 plates. 

 a' 5"60 cm., 8 plates. 

 a" 8-60 cm., 12 plates. 



Owing to the number of reflections (24 faces in the last case), 

 the image gradually became more colored and less intense, so 

 that the adjustment was gradually less certain. At the same 

 time the shift of ellipses was less sensitive. It was, however, 

 wholly the dimness of the interferences which made the adjust- 

 ment more difficult, a result which could have been avoided 

 by using a single plate of thick glass (about 9 centimeters) ; 

 but this was not at hand. Finally a few adjustments of the 

 Fraunhofer mirror were made for incidental reasons not con- 

 nected with the precision of motion of the screw (as shown at 

 A in figure). In other words, through the three inches of 

 screw used no effect of flexure or other serious discrepancy 

 could be detected, the ellipses returning in full strength after 

 each complete turn. 



The fluctuation of the curve which represents the equivalent 

 of 1/36 inch in centimeters {i.e. here -0705 centimeter), is 

 largely within *0005 centimeter at the beginning of the work, 

 when seeing was good. This is about one division of the 

 drum of the micrometer screw, or about 1/141 of the pitch of 

 the screw No. 5 and is largely referable to the difficulty of set- 

 ting the head of the new screw at zero, the disc and gradua- 



