MICROMETER. 
account of several curious observations 
which Mr. Gascoigne made by the help of 
his micrometer, particularly in measuring 
the diameter of the moon and other planets, 
may be seen in the Philosophical Trans- 
actions, Vol. xlviii. 
De la Hire, in a discourse on the mra of 
the inventions of the micrometer, pendu- 
lum clock, and telescope, read before the 
Royal Academy of Sciences in 1717, makes 
M. Huygens tlie inventor of the microme- 
ter. That author, he observes, in his “ Ob- 
servations on Saturn’s Ring, &c.” published 
in 1659, gives a method of finding the 
diameters of the planets by means of a 
telescope, viz. by putting an object, which 
he calls a virgula, of a size proper to take in 
the distance to be measured, in the focus of 
the convex object glass ; in this case, says 
he, the smallest object will be seen very 
distinctly in that place of the glass. By such 
means, lie adds, he measured the diameter 
of the planets as he there delivers them. 
De la Hire says, that there is no method 
more simple or commodious for observing 
the digits of an eclipse, than a net in the 
focus of the telescope. These, he says, 
were usually made of silken threads ; and 
for this particular purpose six concentric 
circles had also been used, drawn upon oil- 
ed paper ; but he advises to draw the 
circles on very thin pieces of glass, with the 
point of a diamond. He also gives some 
particular directions to assist persons in 
using them. In another memoir, he shews 
a method of making use of the same net for 
all eclipses, by using a telescope with two 
object-glasses, and placing them at different 
distances from each other. 
M. Cassini invented a very ingenious 
method of ascertaining the right ascensions 
and declinations of stars, by fixing four 
cross hairs in the focus of the telescope, 
and turning it about its axis so as to make 
them move in a line parallel to one of tliem. 
But the later improved micrometers will 
answer this purpose with greater exactness. 
Dr. Maskelyne has published directions for 
the use of it, extracted from Dr. Bradley’s 
papers, in the Philos. Trans. Vol. Ixii. 
Dr. Derham tells us that his micrometer 
is not put into a tube, as is usual, but is con- 
trived to measure the spectres of tire sun on 
paper, of any radius, or to measure any 
part of them. By this meaus he can easily, 
and very exactly, with tire help of a fine 
thread, take the declination of a solar spot 
at any time of tire day ; and, by his half- 
seconds watch, measure the distance of the 
spot from either limb of the sun. 
J. A. Segner proposed to enlarge the 
field of view in these micrometers, by mak- 
ing them of a considerable extent, and hav- 
ing a moveable eye-glass, or several eye- 
glasses, placed opposite to different parts 
of it. He thought, however, that two would 
be quite sufficient, and he gives particular 
directions how to make use of such micro- 
meters in astronomical observations. 
A considerable improvement in the mi- 
crometer was comnirmicated to the Royal 
Society, in 1743, by Mr. S. Savary; an 
account of which, extracted from the 
minutes by Mr. Short, was published in the 
Philos. Trans, for 1753. The first hint of 
such a micrometer was suggested by M. 
Roemer in 1675 : and M. Bouguer propos- 
ed a construction shnilar to that of M. Sa- 
vary in 1748. The late Mr. Dolland made 
a further improvement in this kind of mi- 
crometer, an account of which was given to 
the Royal Society by Mr. Short, and pub- 
lished in the Philos. Trans, Vol. xlviii. In- 
stead of two object-glasses he used only 
one, which he neatly cut into two semi- 
circles, and fitted each semi circle in a 
metal frame, so that their diameters sliding 
in one another, by means of a screw, may 
have their centres so brought together as to 
appear like one glass, and so form one 
image ; or by their centres receding, may 
form two images of the same object ; it be- 
ing a property of such glasses, for any seg- 
ment to exhibit a perfect image of an object, 
although not so bright as the whole glass 
would give it. If proper scales are fitted to 
this instrument, shewing how far tlie centres 
recede, relative to the focal length of the 
glass, they will also shew how far the two 
parts of the same object are asunder, rela- 
tive to its distance from the object-glass ; 
and consequently give the angle under which 
the distance of the parts of tliat object are 
seen. This divided object-glass micrometer, 
which was applied by the late Mr. Dolland 
to the object end of a reflecting telescope, 
and has been with equal advantage adapted 
by his son to the end of an achromatic 
telescope, is of so easy use, and affords so 
large a scale, that it is generally looked 
upon by astronomers as the most convenient 
and exact instrument for measuring small 
distances in the heavens. However, the 
common micrometer is peculiarly adapted 
for measuring differences of right ascension, 
and declination of celestial objects, but less 
convenient and exact for measuring their 
absolute distances ; whereas the object-glass 
micrometer is peculiarly fitted for measur- 
ing distances, though generally supposed 
