Mother-qf^Peitrl Micrometeu 49 



In tlie winter oK 1805, when 1 was employed in deline-. 

 ating the surface nf the moon, I wished to measure the dia- 

 meter of the lunar spots by applying Mr. Cavallo's niioro- 

 ttietcr to a thirty-inch achromatic ttlescope made liy Berge. 

 But as the eve-piece was moved by a rack and pinion, and 

 consequently could not turn round its axis, the micrometer 

 roust have remained stationary, and could only measure an^ 

 gles in one direction. This difficulty, indeed, might have 

 been surmounted by a mechanical contrivance for turnino: 

 the diaphragm about its centre, or more simply by giving 

 a motion of rotation to the tube which contains the third 

 and fourth eyeglasses. Such a change in the eye piece, 

 however, was both inconvenient and difficult to be made. 

 Mr. Cavallo's micrometer, therefore, has this great disad- 

 vantage, that it cannot be used in reflecting telescopes, or 

 in any achromatic telescope v\here the adjustment of the 

 eye-piece is effected by rack-work, unltss the structure of 

 these instruments is altered for the purpose. Another dis- 

 advantage of this micrometer arises from the slip of mother- 

 of-pearl passing through the centre of the field. The pic- 

 ture in the focus of the eye-glass is broken into two parts, 

 and the view is rendered still more unpleasant by the in- 

 equalitv of the segments into which the field is divided. In 

 addition to these disadvantages, the different divisions of the 

 micrometer are at unequal distances from the eye-glass 

 which views them, and therefore can neither appear equally 

 distinct nor subtend equal angles at the eye. 



Finding that Mr. Cavallo's instrument laboured under 

 these imperfections, I thought of a circular mother-of-pearl 

 micrometer which is free from them all, and has likewise 

 the advantage of a kind of diagonal scale, increasing in ac- 

 curacy with the angle to be measured. This micrometer, 

 which I got executed by Miller and Adie, optical instru- 

 ment makers in Edinburgh, and which I have often used, 

 both in determining sn^all angles in the heavens and such as 

 are subiendcd by terrestrial objects, is represented in fig. 1, 

 which exhioits its appearances in the focus of the fourth 

 eye-glass. The black ring, which forms part of the figure. 

 Vol. 29. No. 113. Oct, 1&07. D ie 



