﻿the Micro- Azimometer. 391 



eyepiece of 16 mm. equivalent focus is replaced by a 

 cylindrical lens. 



11. Incidentally it mny be remarked that the exchange of 

 one cylindrical eyepiece for another of different focal length 

 changes the brightness at any given point of the diffraction 

 pattern inversely as the change of width of the pattern, the 

 total brightness remaining unchanged for the whole pattern, 

 and likewise unchanged for any defined fractional parts 

 thereof, such as the residual strips of light which adjoin the 

 wire when the micrometer is suitably adjusted and set. 

 Hence, when a cylindrical lens serves as eyepiece, it does 

 not appear that any disadvantage on the score of dimness 

 should arise from the use of a fairly high lateral magnifi- 

 cation. 



12. When the slit is illuminated by white light, it is, of 

 course, essential that the object-glass in front of the mirror 

 be achromatic, if close readings of: azimuth are to be possible; 

 but, even so, since each homogeneous component of the 

 pattern is on a scale proportional to the wave-length, the 

 conditions must be far removed from those represented in 

 fig. 2. There are, of course, no lines of absolute blackness 

 corresponding to the points A, B, but when the familiar 

 pattern is regarded from this point of view, it is to me 

 surprising that so passable an appearance of blackness is 

 presented on either side of the central bright band. The 

 consistency with which the wire can be set centrally upon 

 the pattern is certainly far beyond what I expected to be 

 attainable with white light. Moreover, the dimness of the 

 " residual strips " of light observed was distinctly felt to be 

 a difficulty, and so it may well be that appreciably better 

 results could be obtained with a source of white light more 

 brilliant than an acetylene flame. Mr, S. D. Chalmers has 

 suggested to me that colour may play an important pirt 

 in aiding the judgment of the eye, for necessarily the 

 " residual strips" differ from one another in colour as well 

 as in general brightness when the setting is not quite central. 

 I can by no means say that the effect referred to is inopera- 

 tive, but only that I am not aware, of a difference in colour 

 when the setting is only just perceptibly imperfect. 



13. Referring to the light-distribution curve (fig. 2) one 

 would be inclined to think that, in judging ©£ the relative 

 brightness of the strips represented by AED, FGB, the eve 

 must be considerably distracted by the greater total illumi- 

 nation of the adjoining subordinate bands 1\A. BS : and 

 hence it might be expected that a marked advantage would 

 be gained by the use of shutters extending from NA toward.- 



