Two new Micrometers. 425 
The method of adjufting and ufing the catoptric mi- 
crometer is too obvious to require any explanation : it is 
only neceflary to obferve that, befides the table for re- 
ducing the revolutions and parts of the fcrew to minutes, 
feconds, See. it may require a table for correcting a very 
fmall error which arifes from the excentric motion of the 
half mirrors. By this motion their centers of curvature 
will (when the angle to be meafured is large) approach a 
little towards the large mirror; the equation for this pur- 
pofe in fmall angles is infeniible, but when angles to be 
meafured exceed ten minutes, it fhould not be negleCted. 
Or, the angle meafured may be corrected by diminifhing 
it in the proportion the verfed fine of the angle mea- 
fured, fuppofing the excentricity radius, bears to the focal 
length of the fmall mirror. 
The telefcope to which the catoptric micrometer is 
applied is of the Cassegrain conftruCtion. The great 
fpeculum is about twenty-two inches focus, and bears an 
aperture of 5-i inches, which is conderably larger than 
thofe of the fame focal length are generally made : in- 
deed, the apparent utility of this micrometer makes me 
wifli to fee the reflecting telefcope meet with further 
improvements. I believe it would more tend to the ad- 
vancement of the art of working mirrors, if writers on 
this fubjeCt, inftead of giving us their methods of working 
ima- 
