420 Mr. ramsden’s Description of 
rally ufecl in the reflecting telefcope, which ft about as 
i to 4, the aberration in the Cassegrain conftrudtion 
will be to that in the Gregorian as 3 to 5. 
I have mentioned t’nefe circumftances in hopes of re- 
commending the demonftration of curves fuited to the 
purpofes of optics to the attention of mathematicians, 
which would be of great ufe to artifts. 
I fhall conclude this paper with the defcriptionof anew 
micrometer fuited to the principle of refraction ; being 
fenlible that both principles have their peculiar advan- 
tages. Though the former part of this paper proves my 
partiality to the principle of reflection applied to mLro- 
meters, yet the very favourable opinion I have of tl 2- 
fradting telefcope made me attentively confider feme 
means of applying a micrometer to it, which might 
obviate the errors complained of in the former part of 
this paper. 
The application of any lens or medium between theob- 
jedt glafs and its focus muft inevitably deftroy thediftindt- 
nefs of the image ; I therefore have employed for the mi- 
crometer glafs one of the eye glafles requifite in the com- 
mon conftrudtion of the telefcope ; but if it fhould be found 
neceffary to apply an additional eye glafs for the conve- 
niency of enlarging the fcale, I am able thereby to cor- 
4 redt 
