ofihe lute Mr. Ramsden. 237 



of a line to measure the pencil or image of the object-glass. 

 This was tiie original invention, but it was afterwards much 

 improved. 



The pvrometer, destined to measure the dilatation of bodies 

 by heat, afforded exercise also for the talents of Mr. Rams- 

 den ; and with the happiest success, as may be seen in the 

 Philosophical Transactions for 1/85. JNlr. Ramsden, on 

 examining the pvrometer then in use, had observed the ra- 

 dical defect of that instrument, in which the bodies subjected 

 to experiment were not sufficiently separated. But. vith his 

 microscopic pyrometer he found means to compare the na- 

 tural state of a body with the same body exposed to any de- 

 gree of heat or of cold, and by a micrometer adapted to the 

 imcrosGope he measured these variations with an exactness 

 before unknown, and which fiu'nished the measurement of 

 a base with a precision ten times greater than in any of those 

 ever before measured. On this occasion, as on all others, 

 Mr. Ramsden showed a natural saoacity in discovering the 

 essential faults of every instrument, and in inventing the 

 most simple and most exact methods of correcting them. 



Optics arc no less Indebted to him. He found means to 

 correct the aberration of sphericity and refrangibllity in com- 

 pound eve-iilasses applied to all astronomical instruments, 

 and in a new and perfect manner. Opticians had imagined 

 that this could be accomplished by making the image of the 

 object-glass fall between the two eye-glasses; which was 

 attended with this great inconvenience, that the eye-glass 

 could not be touched without deransiinil the line of colllma- 

 tion, and the value of the parts of the micrometer. To re- 

 medy this mconvenience Mr. Ramsden set out from a very 

 simple experimciit, namely, that the edges of an image ob- 

 served through a prism are less coloured according as the 

 image is nearer the prism; and, in consequence of this truth, 

 he sought for the means of placing the two eye-glasses be- 

 tween the image of the object-glass and the eye, without 

 falling to correct the two aberrations, which he did by 

 changing the radii of the curves, and placing the glasses in a 

 manner altogether different from that commonly employed, 

 lie invented also a reflecting object-glass micrometer, a 

 description of which maybe seen in the Transactions of the 

 Jioyal Society of London for 1779. In bis paper on this 

 subject he points out the defects and inconveniences of that 

 of Bougutr, first invented m 1748, in which the different 

 positions of the. eye, in regard to the pencil of light, cause 

 the two images to appear sometimes to touch Cuch other, 

 sometimes to be sep;. rated, and sometimes alternately by a 

 • VoL.XVT. No.tiy. K sort 



