﻿the Micro- Azimometer. 

 Table II. 



Ordinary eyepiece of 16 mm. equivalent focal length. 

 Effective surface of mirror 4 mm. X 4 mm. 

 Object-glass and illumination as in Table I. 



393 



Analysis of 7 



successive 



settings. 





In position of reflected image : — 

 (») In millimetres 



Greatest 



deviation 



from mean. 



Quadratic 

 mean devia- 

 tion from 

 mean. 



Probable 



error of a 



single setting. 



•001?5 

 1/122 

 0^25 



•00091 

 1/180 

 017 



•00062 

 1/266 

 0-114 



(h) In fractional parts of the 1 

 width * of the central l 



In azimuth of mirror (seconds) ... 



* Calculated for \ D . 



15. Trials with homogeneous light have not yet been 

 made, as no sufficiently brilliant source has been available. 

 Coloured glasses were tried, but the general loss of brightness 

 far outweighed the advantage of approximate monochro- 

 matism. 



16. A plane surface, reflecting light of wave-length A, is 

 generally considered sensibly true when its greatest devia- 

 tion from planeness is within A/8, corresponding to an 

 extreme discrepancy of A/4 in retardation. Once this degree 

 of correctness has been attained, resolving-power, or sharpness 

 of definition at a given focal length, cannot be substantially 

 increased by a further approach to absolute flatness. But 

 when we are attempting to discriminate azimuths with a 

 fineness some hundreds of times as great as that which 

 corresponds to the resolving-power, the demands upon the 

 optical quality of the mirror and object-glass are necessarily 

 far heavier. It is the mirror which presents difficulty ; ite 

 necessary lightness in most suspensions rendering it difficult 

 to ensure accurate figuring. Some suggestions on this and 

 related points are contained in §§22,23 below ; meanwhile. 

 it may be mentioned that a home-made mirror of iridio- 

 platinum could have its azimuth fixed with an error which in 

 no case exceeded 05 second, the mirror being ;)■[) mm., 

 wide x 3*1 mm. high, with a thickness of 0'23 mm. In this 

 ease the source of light was the flame of a paraffin lamp. 



