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 refractions. 
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 Abbe 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 
* I have heard, since writing this paper, that it is now at the P^Iacfarlane 01^ 
servatovy at Glasgow. 
