THE 



SCIENTIFIC PEOCEEDINGS 



OF THE 



ROYAL DUBLIN SOCIETY. 



I.— ON A NEW FORM OF GHOST MICROMETER FOR USE 

 WITH ASTRONOMICAL TELESCOPES, by CHAS. E. BUR- 

 TON, F.R.A.S., AND HOWARD GRUBB, f.r.a.s. Plates 1, 2, 



3, AND 4. 



[Read, November 15th, 1880.] 



Before describing the special and novel form of micrometer "we 

 have to introduce to your notice this evening it may be well to 

 mention very briefly the various existing forms which this in- 

 strument usually assumes, and note the special points in which 

 it fails to fulfil its requirements with as much satisfaction as 

 desired. 



The ordinary micrometer in general use consists essentially of 

 a metallic box attached to the draw-tube of the telescope, in which 

 are one or two frames or forked-shaped pieces sliding in carefully- 

 planed grooves, actuated by a fine screw and micrometer head, 

 and carrying spider's webs or other very fine lines. These lines 

 are so placed as to be exactly in the focal plane of the objective. 

 When, therefore, the eye-piece is focussed on any image formed 

 by the objective of the telescope it is also in focus for the wires 

 or webs of the micrometer. According as the micrometer has 

 one or two movable wire-frames and screws it is termed a unifi- 

 lar or bifilar. In order to see these fine lines at night it is 

 almost always necessary to illuminate them in some way. There 

 are two distinct methods of doing this, viz. : — 



1st. By throwing a little false light into the field, in which 

 SciBN, Pkoc, R.D.S, Vol. hi., Pt. i. B 



