490 Mr. J, Barnes on the Analysis of 
Apparatus. | 
After experimenting some time with an instrument which 
seemed to be particularly sensitive to vibrations, even when 
every precaution was taken to eliminate extraneous dis- 
turbances, a new instrument was constructed. In_ the 
construction of this instrument the essential parts sought after 
were, that the mountings for the plates should be rigid and 
placed on a massive base so that the bands should be perfectly 
steady, and that the movable carriage carrying one plate 
should be capable of a very slow uniform motion always 
remaining parallel to its original positicn, enabling one to 
follow clearly the change from one band to another. 
In working with a Michelson interferometer as made by 
Gaertner & Co., the fringes obtained were very steady, even 
when the instrument rested on a table in the laboratory. I 
took this instrument, stripped it of its mirrors and plates, and 
using the base, carriage, and screw constructed the apparatus 
employed. ! 
The apparatus consists of two plane glass plates 3°9 cms. 
by 2°5 cms. and about*6 cm. thick, each slightly prismatic 
in shape ; the two faces making with one another an angle 
between 1” and 2”. This prevents the interference-bands 
formed in the plates themselves being superimposed 
upon those under observation. Both plates are rigidly 
mounted in brass frames. One frame can be moved about a 
vertical axis and the other about a horizontal axis. For very 
small motions about these axes, so that the silvered surfaces 
may be made perfectly parallel, two glass tubes were bent 
into convenient shapes and clamped to the instrument. Their 
ends resting against a frame are covered with thin sheet 
rubber. To the other ends are attached long rubber tubes 
and these connected with a support. By carefully raising or 
lowering these tubes, which are filled with mercury, the pres- 
sure against the frame being therefore varied, very small 
rotations around either axis are obtained and the surfaces 
thereby placed in perfect adjustment. Fabry and Perot 
employed this method, using water in their tubes instead of 
mercury. The carriage containing one of the frames rests 
upon steel ways, very accurately ground, and is connected 
by means of a small carriage, placed underneath, to a screw 
of 1 mm. pitch. The force being thus applied to the carriage 
in a direction parallel to the motion produces no rocking, as 
is shown by the fact that the fringes always remained in 
adjustment during the motion. 
To turn the screw two handles are on the instrument, one 
for rapid and the other for slow motion. A turn of the first 
