331 0. Barns — The Comparison of Two So°ews. 



Fig. 1. 



CflQ. Z. 



A3 



order that they may be identically reversed. Let S be the pitch 

 of the screw at 2¥, for instance, s the corresponding equivalent 

 (often approximate pitch) at N. Then 



(1) 



S COS a = S COS a' 



Now let the mirrors be reversed, the angles a and a' and the 

 beams m and n being reproduced, in virtue of the fixed direc- 

 tion G- T. In this case 



(2) 

 Hence 



S COS a' = 6' COS a 



(3) & = ss' = s< (1 + Jc). 

 Since Jc is very small 



(4) S = s (1 + k/2) = | (s + s'), nearly. 



In order that the reversal may be properly made, the tablets 

 carrying the micrometers M and JV with the attached mirrors 

 should be truly horizontal, so that merely an adjustment of 

 each in azimuth is necessary. The sharp white line which 

 is the direct reflection from the mirror JV is first restored to 

 the cross hairs of the fixed telescope, after which the reflection 

 from M is i*estored to the same image, the two lines merging 

 into a single vertical sharp line. The slit images should now 



