464 
the dotted line, while a beam of light traverses the substance 
parallel to DZ. 
The piece DE is moved to and fro, as it is coupled with the 
rods DC and O'C. Normal to the plane of the drawing, axes have 
been fixed at O and 0’ in a very strongly constructed frame, 
on which the bed is fastened, along which DE moves. The axis 
in O is rotated by a 3 H.P. motor, so that point A describes a 
circle; B, connected with A by the rod AB, acquires a movement 
backward and forward, which is transferred to C enlarged. 
The piece DF of fig. 1 is shown diagrammatically in fig. 2 seen 
from above on a scale of ~;. In A and B there are bronze shoes 
which can slide along steel guides. All these have been constructed 
with great care, so that a rectilinear, horizontal motion of the 
shoes can be obtained. The rods of the transparent substance, which 
Fig. 2. 
rest on a wooden block which is connected with 4 screws to 
A and B, participate in this motion. 
The Plate, which is annexed to this communication, gives a 
general survey of the apparatus. The thickness of the guides amounts 
to 9 mm., the width to 70 mm., the length to 1.84 m. They rest 
on heavy rectangularly bent pieces, which constitute the sides of 
the bed, and which for greater firmness are connected by very solid 
pieces about half a meter long, which are bent twice rectangularly. 
These pieces are arranged on the lefthand at the bottom side, on 
the righthand on the upper side of the bed, as is to be seen in 
the Plate. The entire length of the upper part of the apparatus is 
2.30 m., the length of the stroke is about 1 meter. 
In order to ensure the regular movement of the apparatus it 
appeared to be necessary to provide it with two fly-wheels, a large 
one seen in the foreground, and a smaller one fastened on the other 
side of the axis of rotation, and just visible on the Plate. The whole 
apparatus it fastened with solid bolts to a granite slab, cemented to 
the large pillar of the laboratory. 
As appears from a consideration of fig. 1, the rate of motion of 
the shoe is variable, with two, slightly differing maxima of velocity, 
