436 
ME. GEORGE W. WALKER ON THE DEPENDENCE OF THE 
(Yr 
index of air at different temperatures and made use of the formula (ll to calculate n. 
Their results are briefly as follows: Loeexz and Benoit obtained a value of a equal 
to the ordinary coefficient of expansion for air, Von Lange obtained a value 
considerably less, while Mascart obtained a value considerably greater. Lorenz 
does not indicate what degree of accuracy he obtained, while BenoIt and Von Lange 
do not appear to have obtained as great accuracy as Mascaet. 
Mascaet experimented on a number of gases, and in almost every case obtained a 
value of a appreciably greater than the corresponding coefficient of expansion of the 
gas. This range of temperature was from about 5° C. to 40° C. 
llie disagi'eement between the results of the above-mentioned experimenters in the 
case of air, and the somewhat limited range of temperature used by Mascaet, led me 
to think that a repetition of the experiments on a few gases would be of value. I 
set myself the task of obtaining an accuracv 
of 1 in 500 over a range of temperature from 
10° C. to 90° C., and I think the results 
show that this accuracy has been attained 
and in some cases surpassed. 
The method used was Jamin’s interference 
method, which I shall briefly describe, 
although it is well known (see fig. 1). The 
rays of light from a monochromatic flame 
fall on a thick glass block, whose faces are 
optically plane and parallel, the back face 
being silvered. Two parallel beams of light 
ai'e thus produced and proceed througli the 
two tubes filled witli the gas and reach a 
second block of glass identical with the first. 
The. two Ijeams unite oii emerging from the second glass block and produce 
intfc.i fei ence liands, which may be observed through a telescope. AVhen the jiressure 
in one of the tubes is altered, the bands move across the field of view. As will be 
pioved latei, the number of bands displaced for a given difference of pressure enables 
us to calculate the refractive index of the nas. 
The glass blocks which I used were made by Beinfeldee und Heetel in Munich, 
ihe dimensions were 6X4X3 centims., the faces, 6x3, being optically plane and 
[larallel. One of tlie blocks was placed on an adjustable screw stand, so that the 
necessary adjustments might be made. The other block was placed on a heavy block 
of hard wood. 
Ihe tubes for holding the gas were made of brass, and were about 100 centims. 
long and 1 centini. diameter, and had soldered to them at each end a stuffing hox B 
(see fig. 2). Tlie tidies were soldered to an outer jacket E, which was also made of 
hi ass, and Avas tightly wound on the outside Avith a thick layer of cotton aa ooI. The 
Fig. 1. 
