Meafuremeni of a Bafe on Hoiin flow- Heath. 4^0 



their ufual pofitlon, the image from the objed: lens there beincr 

 formed between the two,, that the difperfioii of rays in the firft 

 may be correcled by that of the fecond. But although this con- 

 fl;ruclion ferves perfectly well every purpofe of the fixed micro- ^ 

 fcope, yet it eould not anfwerin the moveable one, to which the 

 micrometer is attached, where equal parts of an image, or their 

 motion, are to be meafured by the equable motion of the ob-- 

 ieil lens, as fhewn by the micrometer : for in that cafe, the- 

 interpofition of an eye-glafs before the image was formed,- 

 would not only have diminiilied its lize, and thereby rendered 

 the meafure lefs accurate ; but likewife, by refrading the 

 oblique pencils more than thofe nearer the center, it would 

 have defiToyed the equality of the fcale, and made equal parts 

 of the objecb itfelf to have been reprefented unequally in the 

 magnified image, and confequently erroneoufly meafured by 

 unequal parts of the micrometer. It was to remedy a defeft 

 of this fort that Mr. Ramsden propofed his new fy ftem of 

 eye-glafles, defcribed in the Philofophical Tranfa(£lions, vol. 

 LXXIII. 1783, N° 5. And he has here applied that fyflem 

 in the conflru<flion of the micrometer microfcope ; where it? 

 will be perceived, that both glaffes ftand between the eye and 

 the image, whereby the greater magnitude of this lafl is ob* 

 vioufiy preferved, as well as the juft iixjiilarity of all its parts 

 to thofe of the objed itfelf. 



With regard to the fcale of the pyrometer, it is, in the firfl 

 place, to be obferved, that the head of the micrometer fcrew, 

 which is nine-tenths of an inch in diameter, is divided into- 

 fifty equal parts, each of which being reckoned two, it is 

 therefore numbered to 100. Fifty-five revolutions of the 

 head, being equal to 0.77175 of an inch, as meafured with- 

 great accuracy by Mr. Ramsden's itraig.ht-line engine, it 



follows y 



