126 A new Instrument for comparing Linear Measures. 
long heen in the habit of making divisions on glass that are very 
clear and minute at the distances of 100ths of a millimeter, and 
even less*. A glass having two or three millimeters with the 
division. of one of them into 100 parts costs at his house, ten 
or twelve francs. 
Some fcreign artists have also succeeded in this kind of 
work. I procured in my travels in Italy, two small discs of glass 
on each of which are two millimeters, the one divided into ten 
and the other into a hundred equal parts. I had them of 
M. Capello of Turin, an artist who is equally celebrated for his 
inventive mind and his ability to execute what he has con- 
ceived, 
} know also at Paris an amateur, M. Le Baillif, who applies » 
all the leisure moments which his situation under government 
affords, to the cultivation of the sciences; and who among his 
other talents possesses that of dividing a right line on glass in a 
rare and remarkable manner. He has had the kindness to trace 
for me on a small dise of glass 21 tenths of a millimeter; ten 
of which are subdivided into L00dths and 200dths. These divi- 
sions of 200dths. are very neat, and perfectly visible with a mi- 
eroscope magnifying 100 times. 
Those persons who wish to possess a comparer like mine, 
ought first to procure one of these glasses on which a right line 
is divided into as many millimeters as they please, and one of 
these millimeters subdivided into 100dths. The first 10, 20, 30, 
&c. strokes of this subdivision into 100, are prolonged, and the 
5, 15, 25th, &c. should be also prolonged, but less than the 
former, in order that the tenths and half-tenths of a millimeter 
may be distinguished at the first glance of the eve. 
The piece of which I have just spoken must he fixed at one 
of the extremities of a brass rule, the lengths of the strokes of 
the divisions being perpendicular to the length of the rule which 
carries at its extremity a steel stud, intended to be put in con- 
tact with the ends of the linear measures which we may have to 
compare. : 
Another fixed stud must be screwed and held very solidly on 
a board or a table which holds the measures and all the ap- 
paratus. 
When we wish to compare any two linear measures, one of 
them must first be placed in such a way that one of its extremities 
* T have a micromcter screw made by this able artist, two decimeters 
in length. He engaged to cut these divisions at intervals of a balf milli- 
meter, and he succeeded so well that the most rigid proofs could not dis- 
cover the least inequality that was sensible throughout the whole 200 di- 
visions. This is one of the most difficult tasks that can be undertaken of 
this kind. 
may 
ee ae ee, A eS a 
* 
