1406 
is adjusted so that the arm turns when the beam has just passed 
the middle of its course. 
7. Improvement of the Velocity Measurement. In the experiments 
with quartz rods the velocity was measured directly according to a 
method which has been described in II, § 11. We have made this 
method simpler and more delicate, and also arranged it so that the 
velocity could be immediately read in every experiment. 
In main lines the arrangement is still the same as represented 
in Il § 11. The sereen with two slits S, and S, used formerly was 
however, replaced by a screen S (cf. figure of the preceding §), the 
construction of which will be further explained by referring to figure 2. 
Fig. 2. 
Our earlier slit S, is replaced by a glass scale, slit S, by a small 
aperture. ° : 
The graduated glass scale was obtained by covering a glass plate 
with a soot-layer, and by drawing by the aid of a chisel of the 
width of exactly 1 mm., five lines in the soot-layer at mutual 
distances of exactly 1 mm. 
Then the soot-layer was fixed with a drop of varnish, and the 
first line was covered with red glass, the others with blue glass. 
All this was cemented on the beam. In fig. 2 the alternate long 
and short lines are indicated under S; the colours render errors of 
front and back in the observation through lenses impossible. In the 
figure the lines are partially dotted, as they are half covered by a 
screen, which can slide to and fro. During the movement of the 
beam in one sense the lower, during the movement in opposite 
sense the upper half of the scale is automatically covered through 
the inertia of the screen. The blocks B, and B, define the extreme 
positions of the screen. 
As we said, our former slit S, has been replaced by the scale 
with the coloured lines; instead of the slit S, there are two fine 
