Dt Bi^ewster’s Description of a Double Image Micrometer, 165 
% The difficulty of obtaining the exact sections of the crys- 
tal that ar6 required, from the want of natural fractures to guide 
the artist in his operations. 
S. The imperfect achromatism of the eye-piece when the 
moveable lens or lenses are placed in different positions. 
4. The inferiority of achromatic to reflecting telescopes, for 
examining minute luminous objects with high magnifying 
powers. 
We believe it is now universally admitted, both among astro- 
nomers and opticians,, that the correction of colour by the best 
achromatic telescopes, is too imperfect to allow them to be 
brought into comparison with the fine reflecting telescopes now 
made in England. For the ordinary purposes of astronomy, 
the achromatic telescope possesses peculiar advantages ; but 
when it is directed to minute double stars, to luminous points, 
or to small planetary discs, its performance is inferior to that 
of the reflecting telescope. For these reasons, a Gregorian 
or Cassegrainian telescope is employed ; and the variation of 
the magnifying power is produced in the manner which I have 
described in my Treatise on new Philosophical Instruments, 
namely, by separating the eye-piece from the great speculum, 
atid procuring an adjustment by a motion of the small specu^ 
lum *. 
Having thus obtained what I conceive to be the most perfect 
of all methods of obtaining a variation of the angle, -^a method, 
too, in which the performance of the telescope is in no respect in- 
jured, we may apply it to the various contrivances which have 
been invented for giving double images. 
In adopting the principle of Rochon as one of the best, I 
form the doubly refracting prism out of the colourless Topaz of 
New Holland^ which is much freer from veins and imperfections 
of crystallization than the purest rock-crystal, and has also the 
advantage of a lower dispersive power. In certain sections of 
the crystal, when we require only a very small separation of the 
* It maybe proper to mention, that an oversight’ occurs in the investigation of 
the hature of the scale of this micrometer, which the mathematical reader wHl 
mediately discover. 
