118 
PHYSICS: C. BARUS 
produce the same phenomena. The flint plate used, however, proved 
to be inadequately plane, so that the result is in doubt. 
More recently I have endeavored to secure similar results by submerg- 
ing the lens (convex or concave) in a Hquid of about the same index 
of refraction. This method would seem to be interesting in other 
respects; for it is probable that the index of the solid may be deter- 
mined in this way irrespective of form.^ If for instance the liquid 
and the solid have the same index, one would be tempted to infer that 
the latter may be removed or inserted, without displacing the center 
of ellipses at the particular wave length under consideration. The index 
of the liquid in place is then also determinable by the interferometer, 
to a few units in the fourth place. Unfortunately the problem is not 
so simple. 
If experiments of the present kind are to be accurate, it is obvious 
that the walls and cavity of the trough in which the lenses are to be 
submerged must be optically plane-parallel. Otherwise some compen- 
sating adjustment must be made at the opaque mirrors of the inter- 
ferometer and for this no adequate allowance can be made. It did not 
however seem worth while to provide expensive apparatus, before the 
method had been worked out in detail. Accordingly the present ex- 
periments were conducted with troughs of ordinary plate glass put 
together by myself, and little attention was given to absolute values 
of index of refraction, as such. 
2. Preliminary experiments. — The first experiments were made on a 
large linear interferometer with distances of nearly 2 meters between 
the mirrors. The rays in such a case are all very nearly parallel. Sun- 
light, arc light or the Nernst filament were each available for illumina- 
tion. With a very long collimator (2 meters) and wide single lens ob- 
jective (10 cm. or more), the Nernst filament may be used directly in 
place of the slit. 
On inserting the trough with a thickness of 1.3 cm. of CS2 solution 
normally into one component beam, the original very large ellipses were 
reduced in size and rounded as usual to smaller circles. Submerging 
a convex lens (1 diopter) into the liquid until the beam passed symmetri- 
cally through it, changed these circles to very long horizontal spindles. 
A concave lens similarly produced horizontally very eccentric hyperbolae. 
All these fringes lie considerably in front of the principal focal plane of 
the telescope and the abnormal forms are necessarily relatively faint. 
They change in shape and intensity with the focal plane observed. 
On mixing CS2 with kerosene (about equal parts) types of fringes 
shown in figure 1, but with many more lines, were obtained. This is 
