164 Dr Brewster's Description of a Double Image Micrometer. 
that there may be other insects hearing nothing in common with 
us, but endued with a power of exciting, and a sense that per- 
ceives vibrations of the same nature indeed as those which con- 
stitute our ordinary sounds, but so remote, that the animals who 
perceive them may be said to possess another sense, agreeing 
with our own solely in the medium by which it is excited, and 
possibly wholly unaffected by those slower vibrations of which 
we are sensible. 
Akt. XXVI. — Description of a New Double Image Microme- 
ter for Measuring the Diameter of Minute Celestial Objects. 
By David Brewster, LL. D., F. R. S. Lond. & Ed. &c. 
Every double image micrometer in which the angle is va- 
ried by optical means, must consist of two separate parts, one of 
which prpduces the duplication of the image, while the other va- 
ries the magnifying power of the telescope, and thus separates 
the two images, or causes them to approach, till an accurate 
contact is obtained. 
In the micrometer described by the Reverend Dr Pearson, 
(see this Journal^ Vol. III. p. 189,-190.) the double image is 
produced by a prism of rock-crystal placed between the first 
eye-glass and the eye, and the variation of the angle is effected 
by separating the lenses of which it consists. We do not know 
what part of this instrument Dr Pearson claims as his own ; but 
the variable eye-piece which he uses, was invented by me in 
1805 ; and the prism of rock-crystal is the undoubted invention 
of that ingenious and amiable man the late Abbe Rochon, who 
used it both at the object-end of the telescope, in the middle of 
the telescope, and at the eye-end of the instrument. 
The objections to the eye-piece micrometer described by Dr 
Pearson, may be ranked under three heads : 
1, The imperfectly crystallised state of quartz, which is ne- 
ver perfectly homogeneous, and in which minute lines and veins 
of different refractive powers may be always seen by proper pre- 
cautions. 
