Meafuremeni of a Bafe on Hounflow-Beath, 469 
their ufual pofition, the image from the object lens there being 
formed between the two, that the difperfion of rays in the firft 
may be corrected by that of the fecond. But although this con- 
ftru&ion ferves perfectly well every purpofe of the fixed micro- 
fcope, yet it could not ahfwer in 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- 
ject lens, as lhewn by the micrometer : for in that cafe, the 
interpolation of an eye-glafs before the image was formed, 
would not only have diminiihed its fize, and thereby rendered 
the meafure lefs accurate ; but likewife, by refracting the 
oblique pencils more than thofe nearer the center, it would 
have- deft royed the equality of the fcale, and made equal parts 
of the objeCt 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 defeCt 
of this fort that Mr. Ramsden propofed his new fyftem of 
eye-glafles, defcribed in the Philofophical TranfaCtions, vol. 
LXXIII. 1783, N° 5. And he has here applied that fyftem. 
in the conftruCtion 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 laft is ob» 
vioufiy preferved, as well as the juft fiupftlarity of all its parts 
to thofe of the objeCt itfelf. 
With regard to the fcale of the pyrometer, it is* in the firfb 
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 ftraight-line engine, it 
follows.* 
