VOL. LXXXI.] PHILOSOPHICAL TRANSACTIONS. 77 



angle, must first of all unscrew the tube, which contains the eye-glass and mi- 

 crometer, from the rest of the telescope, and, looking through the eye-glass, 

 must place the micrometer where the divisions of it may appear quite distinct to 

 his eye. If any person should not like to see always the micrometer in the field 

 of the telescope, then the micrometrical scale, instead of being fixed to the dia- 

 phragm, may be fitted to a circular perforated plate of brass, wood, or even 

 paper, which may be occasionally placed on the said diaphragm. 



Mr. C. has made several experiments to determine the most useful substance 

 for this micrometer. Glass, which he had successfully applied for a similar pur- 

 pose to the compound microscope, seemed at first to be the most promising ; but 

 it was at last rejected after several trials : for the divisions on it generally are 

 either too fine to be perceived, or too rough ; and though with proper care and 

 attention the divisions may be proportioned to the sight, yet the thickness of 

 the glass itself obstructs in some measure the distinct view of the object. Ivory, 

 horn, and wood, were found useless for the construction of this micrometer, on 

 account of their bending, swelling, and contracting very easily ; whereas mother 

 of pearl is a very steady substance, the divisions on it may be marked very 

 easily, and, when it is made as thin as common writing paper, it has a very 

 useful degree of transparency. It is something less than the 24th part of an 

 inch broad ; its thickness is equal to that of common writing paper ; and the 

 length of it is determined by the aperture of the diaphragm, which limits the 

 field of the telescope. The divisions on it are the 200ths of an inch, that 

 reach from one edge of the scale to about the middle of it, excepting every 5th 

 and 10th division, which are longer. The divided edge of it passes through the 

 centre of the field of view, though this is not a necessary precaution in the con- 

 struction of this micrometer. Two divisions of the above described scale in the 

 telescope are very nearly equal to 1 minute ; and as a quarter of one of those di- 

 visions may be very well distinguished by estimation, therefore an angle of 4. 

 part of a minute, or of T'\, may be measured with it. 



When a telescope magnifies more, the divisions of the micrometer must be 

 more minute ; and when the focus of the eye-glass of the telescope is shorter 

 than half an inch, the micrometer may be divided with the SOOths of an inch ; 

 by means of which, and the telescope magnifying about 200 times, one may 

 easily and accurately measure an angle smaller than half a second. On the other 

 hand, when the telescope does not magnify above 30 times, the divisions need 

 not be so minute : for instance, in one of Dollond's pocket telescopes, which 

 when drawn out for use, is about 14 inches long, a micrometer with the lOOths 

 of an inch is quite sufficient, and one of its divisions is equal to little less than 

 3' ; so that an angle of a minute may be measured by it. 



