106 
meters of available thickness, if we take the 
power required to resolve the D-lines as 
unity. Thecompound spectroscope had an 
available thickness of 12 inches or 30 cm., 
so that its theoretical resolving power (in 
the yellow region of the spectrum) would 
be about 22. With the aid of a reflector 
the prism could be used twice over, and 
then the resolving power is doubled. 
One of the objections to a stereoscope de- 
pending upon bisulphide of carbon is the 
sensitiveness to temperature. In the ordi- 
nary arrangement of prisms the refracting 
edges are vertical. If, as often happens, 
the upper part of a fluid prism is warmer 
than the lower the definition is ruined, one 
degree (Centigrade) of temperature making 
nine times as great a difference of refrac- 
tion asa passage from D, to D,. The ob- 
jection is to a great extent obviated by so 
mounting the compound prism that the re- 
fracting edges are horizontal, which,of course, 
entails a horizontal slit. The disturbance 
due to a stratified temperature is then 
largely compensated by a change of focus. 
In the instrument above described, the 
dispersive power is great—the D-lines are 
seen widely separated with the naked eye— 
but the aperture is inconveniently small 
(4-inch). In the new instrument exhibited, 
the prisms (supplied by Messrs. Watson) 
are larger, so that a line of ten prisms occu- 
pies 20 inches. Thus, while the resolving 
power is much greater, the dispersion is 
less than before. 
In the course of the lecture the instru- 
ment was applied to show the duplicity of 
the reversed soda lines. The interval on 
the screen between the centers of the dark 
lines was about half an inch. 
It is instructive to compare the action of 
the glass powder with that of the spectro- 
scope. In the latter the disposition of the 
prisms is regular, and in passing from one 
edge of the beam to the other there is com- 
plete substitution of liquid for glass over the 
SCIENCE, 
[N. S. Vou. X. No. 239. 
whole length. For one kind of light there 
is no relative retardation, and the resolving 
power depends upon the question of what 
change of wave-length is required in order 
that its relative retardation may be altered 
from zero to the quarter wave-length. All 
kinds of light for which the relative retar-- 
dation is less than this remain mixed. In 
the case of the powder we have similar 
questions to consider. For one kind of 
light the medium is optically homogeneous,. 
i. €., the retardation is the same along all 
rays. If we now suppose the quality of the 
light slightly varied, the retardation is no 
longer precisely the same along all rays; but 
if the variation from the mean falls short of 
the quarter wave-length it is without impor- 
tance, and the medium still behaves prac- 
tically as if it were homogeneous. The 
difference between the action of the powder 
and that of the regular prisms in the spec- 
troscope depends upon this, that in the lat- 
ter there is complete substitution of glass for 
liquid along the extreme rays, while in the 
former the paths of all the rays lie partly 
through glass and partly through liquid in 
nearly the same proportions. The difference: 
of retardations along various rays is thus a. 
question of a deviation from an average. 
It is true that we may imagine a relative 
distribution of glass and liquid that would. 
more nearly assimilate the two eases. If,. 
for example, the glass consisted of equal 
spheres resting against one another in 
cubic order some rays might pass entirely 
through glass and others entirely through 
liquid, and then the quarter wave-length of 
relative retardation would enter at the same 
total thickness in both cases. But such an 
arrangement would be highly unstable, and 
if the spheres be packed in close order the 
extreme relative retardation would be much 
less. The latter arrangement, for which 
exact results could readily be calculated, 
represents the glass powder more nearly 
than does the cubic order. 
