258 Account of the Life and Labours 



sort of oscillation. He found also that the aberration of the 

 rays, which renders the object badly defined, increased the 

 inconvenience of that instrument. He thought it would 

 therefore be necessary to abandon the principle of refrac- 

 tion, and to substitute that of reflection. This instriniitnt, 

 as simple as ingenious, contains no more mirrors or glasses 

 than what are necessary- for the telescope ; and the separa- 

 tion of the two images depends only on the inclination of 

 the mirrors, and not on the focus. 



He however employed himself in improving the refract- 

 ing micrometer, and conceived the happy idea of placing 

 this micrometer not towards the object-glass, but exactly in 

 the conjugate focus of the first evc-glass. This niicrometyr 

 is composed of t\\ o plano-convex lenses, which can be moved 

 and form two images, as in the cbject-frlass micrometers, but 

 with this difference, that the lavs before they fall on the plano- 

 convex lenses pass through a lens convex on both sides, at 

 a certain distance towards^ the object-glass. By these means 

 the contrary refraction of the two plano-convex lenses, and 

 the convex lens, corrects the error w hich takes place \n 

 object-glass micrometers, where the image depends only on 

 the tocus of the two plano-convex lenses. The image being 

 already considerably magnified before it falls on the refract- 

 ing micrometer, the imperfection of the glasses can occasion 

 only an insensible error in the measurement of angles. It 

 is true, indeed, that by this position the field of the micro- 

 meter will be smaller than what it would be were the micro- 

 meter near the object-glass. Mr. Ramsden devised means 

 also for making the images to be uniformly illuminated in 

 every part of the field. \Vith this micrometer the diameter 

 of the planets may be measured in every direction ; it may 

 be adapted to achromatic telescopes of ever\' kind ; it may 

 be brought near to or removed from the object-glass at plea- 

 sure, to render vision distinct ; and it may be taken from 

 the tube of the eye-glasses, that the telescope may be 

 employed without a micrometer. AH these advantages 

 have given great reputation to Ramsden's micrometers, 

 and the astronomer w ho can obtain one of them is for- 

 tunate. 



In consequence of these and other inventions Mr. Rams- 

 den was elected a fellow of the Royal Society in 1786. 



The objects hitheito mentioned, however, are not the 

 most impoitant of tlie works of Mr. Ramsden. The equa- 

 torial, the transit instrument, and quadrant, received in his 

 hands new improvements. The e (uatorial first constructed 

 by Sisson, and vvhieh was somewhai improved by Short, was 



xiiuch 



