570 
■S T N tt LE - 1, t N S MICRO >I liT E R . 
dry state, with the acids, are for the most part decomposed into 
oxida in icum, and reduced metal. 
I 
IV. 
Description nf a single-lens Micrometer. By WmciAM Hype 
Wollaston, M. D. Sec. R. S. Phil. Trans. 1813, 
A single lens is 
employed to 
niaguity ; 
and the ohjict 
ot'its Hilt mu I 
size is seen hy 
the same eye 
through a per 
t'oratien. 
H aving had orcaslon to measure some very small wires 
wi th .'I greater degree of accuracy than I was enabled to 
do by any instrument hitherto made use of for such purposes, 
I was led to contrive other means that might more effectually 
answer the end proposed. The instrument to which I had 
recourse, is furnished with a single lens of about one-twelfth 
of an inch focal length. The aperture of such a lens is 
necessarily small, so that when it is mounted in a plate of brass, 
a small perforation can be made by the side of it in the brass, 
as near to its centre as one-twerily-tifth of an inch. 
When a lens thus mounted is placed before the eye for the 
purpose of examining any sm.ill object, the pupil is of sufficient 
magnitude for seeing distant objects at the same time through 
the adjacent perforation, so that the apparent dimensions of the 
magnified image might be compaied with a .scale of inches, 
feet, or yards, according to the distance at which it might be 
convenient to place it. A scale of smaller dimensions attached 
to the instrument, will, however, be found preferable on ac- 
count of the steadiness with which the comparison may be 
made; .and it may be seen with sufficient distinctness by the 
naked eye, without any effort of nice adaptation, by reason of 
the smallness of the hole through which it is viewed. 
The construction that I have chosen for the scale, is repre- 
sented in fig. 1. (See Plate VI.) It is composed of small wires, 
about I -50th of an inch in diameter, placed side by side, so as 
to form a scale of equal parts, which may with ease be counted 
by meent ©f a certain regular variation of the lengths of th» 
wiri5«. 
