MICROMETER. 
improper for the former purpose. But Dr. 
Maskelyne has found that this may be ap- 
plied with very little trouble to that pur- 
pose also ; and he has furnished the direc- 
tions necessary to be followed when it is 
used in this manner. The addition requisite 
for this purpose is a cell, containing two 
wires, intersecting each other at right an- 
gles, placed in the focus of the eye-glass of 
the telescope, and moveable round about 
by the turning of a button. For the de- 
scription of this apparatus, with the method 
of applying and using it, see Dr. Maske- 
lyne’s paper on the subject, in tlie Philos. 
Trans. Vol. Ixi. 
After all, the use of the object-glass mi- 
crometer is attended with difficulties, aris- 
ing from the alterations in the focus of the 
eye, which are apt to cause it to give dif- 
ferent measures of the same angle at diffe- 
rent times. To obviate these difficulties, 
Dr. Maskelyne, in 1776, contrived a pris- 
matic micrometer, or a micrometer consist- 
ing of two achromatic prisms, or wedges, 
applied between the object-glass and eye- 
glass of an achromatic telescope, by moving 
of which wedges nearer to or further from 
the object-glass, the two images of an object 
produced by them appeared to approach 
to, or recede from, each other, so that the 
focal length of the object-glass becomes a 
scale for measuring the angular distance of 
the two images. The rationale and use of 
this micrometer are explained in the Philos. 
Trans, vol. Ixvii. 
Mr. Ramsden has described tw’o new mi- 
crometers, which he has contrived for reme- 
dying the defects of the object-glass micro- 
meter. One of these is a catoptric microme- 
ter, which, besides the advantage it derives 
from the principle of reflection, of not be- 
ing disturbed by the heterogeneity of light,- 
avoids every defect of other micrometers, 
and can have no aberration, nor any defect 
arising from the imperfection of materials, 
or of execution ; as the great simplicity of 
its construction requires no additional mir- 
rors or glasses, to those required for the 
telescope ; and the separation of the image 
being effected by the inclination of the two 
specula, and not depending on the focus of 
lens or mirror, any alteration in the eye of 
an observer cannot affect the angle measur- 
ed. It has peculiar to itself the advantages 
of an adjustment, to make the images coin- 
cide in a direction perpendicular to that of 
their motion ; and also of measuring the 
diameter of a planet on both sides of the 
aero j which will appear no inconsiderable 
advantage to observers who know how 
much easier it is to ascertain the contact of 
the external edges of two images than their 
perfect coincidence. 
The other micrometer invented and de- 
scribed by Mr. Ramsden, is suited to the 
principle of refracLion. This micrometer is 
applied to the erect eye-tube of a refracting 
telescope, and is placed in the conjugate 
focus of the first eye-glass, as the image is con- 
siderably magnified before it comes to the 
micrometer, any imperfection in its glass 
will be magnified only by the remaining 
eye-glasses, which in any telescope seldom 
exceeds 5 or 6 times ; and besides, the size 
of the micrometer glass will not be the 
100th part of the area which would be 
required, if it were placed at the object- 
glass ; and yet the same extent of scale is 
preserved, and tlie images are uniformly 
bright in every part of the field of the 
telescope. See Philos. Trans. Vol. Ixix. 
In the Philos. Trans, for the year 1782, 
Dr. Herschel, after explaining the defects 
and imperfections of the parallel- wire micro- 
meter, especially for measuring the appa- 
rent diameter of stars, and the distances 
between double and multiple stars, de- 
scribes one for these purposes, which he 
calls a lamp micrometer ; one that is free 
from such defects, and has the advantage of 
a very enlarged scale. In speaking of the 
application of this instrument, he says, “ It 
is well known to opticians, and others, who 
have been in the habit of using optical in- 
struments, that we can with one eye look 
into a microscope, or telescope, and see an 
object much magnified, while the naked 
eye may see a scale upon which the magni- 
fied picture is thrown. In this manner I 
have generally determined the power of my 
telescopes ; and any one who has acquired 
a facility of taking such observations, will 
very seldom mistake so much as one in fifty 
in determining the power of an instrument, 
and that degree of exactness is fully suffi- 
cient for the purpose. 
“ The Newtonian form is admirably 
adapted to the use of this micrometer, for 
the observer stands always erect, and looks 
in a horizontal direction, notwithstanding 
the telescope should be elevated to the 
zenith. The scale of the micrometer at the 
convenient distance of 10 feet from the 
eye, with the power of 460, is above a quar- 
ter of an inch to a second ; and by putting 
on my power of 932 I obtain a scale of 
more tlian half an inch to a second, without 
increasing the distance of the micrometer ; 
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