﻿394 Dr. C. V. Burton on 



17. A micrometric device, such as that described, would 

 necessarily be inapplicable in certain uses of: reflecting 

 instruments, even though a large gain of sensitiveness might 

 be desirable. For example, the throws of a reflecting 

 ballistic galvanometer could not be so read, unless we were 

 content to set the micrometer laboriously by trial and error, 

 so that, on depressing a key (or what not), the extreme 

 excursion of the mirror just brought about bisection of the 

 diffraction pattern by the wire. 



18. It woald, however, be practicable to deal with changes 

 of azimuth which took place slowly enough to be adequately 

 recorded by readings taken every few minutes. In such a 

 case the micrometer could from time to time be set in 

 advance, and the instant noted when bisection of the 

 diffraction pattern was observed. 



19. But micrometric methods find a more obvious appli- 

 cation where the deflexions to be observed are steady, or so 

 slowly changing as to allow of a setting being rendily made 

 at any given time. In many cases the instrument to be read 

 (say, a radio-micrometer, or a galvanometer connected to a 

 linear thermopile) would be furnished with a relatively stiff 

 suspension, so that the greatest deflexion to be observed 

 would come within the range of the micrometric traverse. 

 If this were 25 mm., there should be no difficulty in reading 

 deflexions correct to 1/25000 part of the maximum deflexion 

 observable * ; a single micrometer screw of 0'5 mm. pitch, 

 with head divided into 50 parts, should suffice. Incidentally 

 there might be the advantage of a reduced instrumental 

 period. 



20. Above all, reflecting instruments used as detectors 

 appear to be a promising field for the micro-azimometer, as I 

 have ventured to call it. In place of a micrometer,, an 

 uncalibrated fine-motion screw here suffices to reset the zero 

 from time to time if required. In relative resistance measure- 

 ments it may be said that the limits of attain;) ble accuracy 

 are imposed directly and indirectly by the galvanometer* If 

 we have the means of making the current in the bridge many 

 times less, while the sensitiveness to lack of balance is at the 

 same time greatly increased, a marked improvement in 

 accuracy may be expected. This is especially the case in 

 bolometer and resistance-thermometer work, where the 

 temperatures of certain resistance-wires are the quantities 

 which directly concern us. 



* Always assuming an accurately made (or calibrated) micrometer, 

 and a sensibly flat mirror of rectangular outline. 



