1 'zvo new Micrometers. 421 
affedts the diftindtnefs of the image, but alfo the angle 
meafured by the micrometer. 
At the time I took up this fubjedt, the divided objedt- 
glafs micrometer was the only one which meafured an- 
gles by the reparation of tw r o images. Since that time, a 
very ingenious application of the prifm to this purpofe 
has been invented by the rev. Dr. maskelyne, Aftrono- 
mer Royal; and although experience has not yet afcer- 
tainecl the extent of its merit, it will always deferve great 
confideration from its ingenuity ; but the more I conli- 
dered the fubjedt, I became more fully convinced, that 
the principle of refledtion applied to micrometers would 
have great advantages over thofe hitherto constructed on 
the principle of refradtion ; and the catoptric micrometer 
I have the honour to defcribe, belides the advantage it 
derives from the principle of refledtion, of not being dis- 
turbed by the heterogeniety of light, avoids every defedt 
of other micrometers, and can have no aberration, nor 
any defedt which arifes from the imperfedtion of mate- 
rials, or of execution, as the extreme Simplicity of its 
conitrudtion requires no additional mirrors or glafles to 
thofe required for the telefcope : and the Separation of the 
image being effedted by the inclination of the two Spe - 
cula, and not depending on the focus of<my lens or mir- 
Vol. LXIX. Kick ror. 
