REFRACTIVE INDICES OF SEVERAL SUBSTANCES. 
529 
Sir William Hamilton congratulated 
Mr. Russell upon the most successful 
issue of his researches thus far, and 
strongly urged upon him the continua- 
tion of them, and even the extension of 
of them. The fund upon which Mr 
Russell drew for bearing the expense of 
his experiments, conducted, as they had 
obviously been, upon the most splendid 
scale, of course left nothing to be de- 
sired if it still continued unexhausted ; 
but to him it appeared that if there was 
the least chance of a difficulty in procu- 
ring funds, those of the British Associa- 
tion could not be applied to a more 
legitimate ora more important purpose; 
and he felt little doubt that, if there 
was any necessity for such application, 
it would be successful. 
Professor Powell then read a paper 
respecting the Refractive Indices of se- 
veral substances. 
The Professor commenced by giving 
an explanation of the term Refractive 
Index, and pointed out the changes of 
meaning which it had undergone since 
the time of Newton. He then explained 
what was meant by the “dispersive 
power” of a substance. The determina- 
tion of the refractive indices for definite 
rays of the coloured image of the sun, 
or solar spectrum, marked by the dark 
lines of Wollaston, from direct observa- 
tions of their deviations, produced by 
prisms of various substances, was first 
proposed by Frauenhofer ; and, by the 
aid of instruments of extraordinary de- 
licacy and exactness, executed by him, 
obtained for ten media solid, and was 
carried on by Mr. Rudberg for ten 
more. The absolute necessity of an ex- 
tended series of such determinations, 
%vas pointed out by Sir J. Herschel and 
Sir D. Brewster, and was further urged 
by a special recommendation of the 
British Association; but Mr. Powell, 
finding that no other person was stepp- 
ing forward to undertake the task, 
stated, that he was himself most reluc- 
tantly induced to endeavour to make 
some progress in this matter, so highly 
important in a practical point of view. 
The apparatus of Frauenhofer, besides 
its extreme complexity, required in the 
using, a skill and accuracy, possessed by 
few but himself. This, and other con- 
siderations induced the learned Pro- 
fessor himself to procure the aid of 
Mr. Simms in constructing a much 
simpler apparatus, the essential parts 
of which are a graduated circle, with the 
prism of its centre diameter 10 inches — 
an achromate telescope, with cross 
wires at its focus — an arm, projecting 
from the centre, carries the prism with 
two motions, one to adjust it to paralle- 
lism, with the slit through which the 
light is admitted, and which is about 
the I-20th of an inch wide, and then 
round its axis. For liquid media, hollow 
jwisms, or troughs, of different angles, 
were provided, whose inclined sides are 
of plate glass, formed with truly parallel 
surfaces and the angles accurately de- 
termined previously. For seeing some 
of the lines of the sjiectrum, the light of 
the sun is required, in which case one or 
more thickness of purple glass is used, 
and, by means of the lines themselves, 
the most perfect parallelism of the prism 
with the slit can be obtained ; and, since 
the lines become indistinct, except in 
or near the position of minimum devia- 
tion, that position is always adopted; 
this last deviation is accurately observed 
by the focal wires. The absolute devia- 
tion is observed directly from the za 
ro point of the circle, or that which 
corresponds with the telescope direct- 
ed to the slit. This instrument, the 
Professor stated, although comparative- 
ly so simple, admitted of a degee of ac- 
curacy, even greater than what he cared 
to wait for obtaining, his object being a 
great number of approximations to the 
truth, rather than the attainment of the 
utmost precision. He then proceeded 
to notice the care required in attending 
to diversities of temperature, since he 
had found the greatest differences to 
result from this source. He alluded 
briefly to the precautions used, and 
then stated the means by which he had 
obtained the oils, alcohol, and chemi- 
cal substances, which were the subjects 
of these experiments, Mr. Allen and 
Professor Daubeny giving him the 
most essential aid in this important part 
of his labours. The fluids that he ex- 
amined were oil of cassia, oil of aniseed, 
sulphuret of carbon, balsam of Peru, 
kreosote, oil of sassafras, oil of pimento 
oil of angelica, sulphuric acid, muriatic 
acid, nitric acid, alcohol 39 =.8 15, solu- 
tion of chromate of lead in nitric acid, 
solution of chromate of potass, of mu- 
riate of lime, muriate of ammonia, ni- 
trate of potass, sulphate of magnesia, 
nitrate of mercury, muriate of barytes^ 
sulphate of soda, muriate of zinc, nitrate 
