THE TEACHING OF MECHANICS BY EXPERIMENT. 
253 
measurement of long lengths is part of his field work in surveying, if 
he is a student of engineering, but the measurement of short lengths 
is part of his laboratory work. He learns how to read off lengths by 
an ordinary scale, estimating to tenths of the smallest divisions marked 
on the scale, how to use beam compasses and calipers, screw microm¬ 
eters working by touch, micrometer microscopes, the spherometer, 
the cathemometer and so forth. He learns to use and test the plani- 
meter as well as to measure areas by other means. He learns^of 
course the use of the balance and how to find specific gravities, fin 
connection with his length measurements he uses an optical lever^such 
as that shown in Fig. 1. 
Fig. 1. Optical Fever. 
This is the same in principle as the optical lever described 
in various text-books on practical physics. A small three-legged 
stool carries a mirror; its two front legs rest respectively in a 
hole and in a slot, while the third leg rests on a plane surface at 
the back. The body whose thickness is to be determined, say a metal 
plate or a piece of glass, is put under the back leg; this tilts the 
mirror, and the amount of the tilting is measured by observing in the 
reading telescope the reflection from a mirror of a fixed vertical scale. 
In this way the student learns to make a survey of the thickness of 
the plate and he compares the thickness so determined with meas¬ 
urements made by means of a screw micrometer and otherwise. It 
will be noticed that this optical lever is entirely self-contained; the 
reading telescope and scale are carried on the same stand as the 
mirror. This is a feature of considerable practical advantage in a 
laboratory, especially when many pieces of apparatus have to be used. 
Where the space is small and the number of students is large it is 
desirable that each piece of the apparatus should be, so far as possible, 
a self-contained whole, which may be at once set on the table ready 
