James Watt (1736-1819)

124

                      Mr Watt on New Micrometers.

ced in the focus of the object-glass of a telescope, two images
were formed of each object, by which its diameter could be
measured. An index, and divided sector of a circle, served to
measure the comparative refraction.

This instrument I made with the sector and radius of wood,
and gave it to Professor Anderson, of Glasgow College, and, I
suppose, it is still among his apparatus, which he left to a pu-
blic, institution*. The Abbé Rochon afterwards published, in
1783, a description of some micrometers with prisms, but I
think they were, upon somewhat different principles in their
construction.

The cross-hair micrometer, as described, leaving me too much
in the power of my assistants, where the distances were greater
than permitted me to read off the number of chains on the rod
myself. I thought of another about 1772 or 1773, which con-
sisted of a telescope with an object-glass of a long focus, say
three or four feet; this was placed in a tube with a slit in one
side of it, nearly as long as the focus of the telescope, and the
object-glass being fitted to a short tube, which slid from end to
end of the slit, could be moved backwards and forwards by
means of a piece of metal fixed to the short tube, and coming
out through the slit; a glass of six to nine inches focus was also
fixed in the outer tube, of the nature of what is called a field
glass, and to this was added an eye-glass, with a cross hair-piece
in its focus.

Now it is evident, that if the object-glass be moved nearer
the field-glass, their common focus will be shortened, and the
image at the cross-hairs diminished proportionally, until the
glasses come into contact, when their common focus will be
shorter than that of the field-glass alone; and two indexes fixed
upon a rod being subtended by the cross-hair at any given dis-
tance, the same rod with its indexes being removed nearer the
observer, upon sliding the object-glass nearer the eye, they may
again be subtended by the cross-hairs, and a scale on the side of
the tube will show the comparative distance they have been re-
moved, and the distance of the first object being known, that of

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* I have heard, since writing this paper, that it is now at the Macfarlane Ob-
servatory at Glasgow.