MR. J. B. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 441 
Experimental determinations of /x.—A large 4-prism spectroscope by Browning was 
used. 
It has a silver circle about 94> inches in diameter or nearly 30 inches in circumference. 
The vernier reads to 10 '. 
The prisms were removed and replaced by one prism of the glass under examination. 
/x was determined in the ordinary way by first measuring the angle of each prism 
and then mapping a spectrum observed through it at minimum deviation. 
An attempt was made to observe the solar spectrum, but owing to the uncertainty 
of the weather in October it was abandoned, and a spark between magnesium poles in 
air was used as the source of light instead. 
The spark was produced by means of my 17-inch coil working with its own vibrating 
break. A Leyden jar was connected to the secondary circuit in the usual way, and a 
convenient discharging frame caused the spark to take place in front of the slit. 
The wires of the primary circuit were brought round to a plug-key fixed to the 
table close to the spectroscope. This enabled the observer to start and stop the coil 
without leaving his place. 
To determine the refractive index for waves of infinite length, we proceed as 
follows :■— 
We ha,ve the general equation 
A , B 
To determine A, a determination of the values of /x for two rays of different wave 
lengths X and X' are necessary and sufficient, for we have 
/xX 2 = AX’-bB 
p/X' 3 =AX' 2 +B 
Subtracting one equation from the other we eliminate B and obtain 
A= 
/Z.A 3 —/xTl' 3 
A 2 -A/ 3 
But wdien X=oo , /x=A. Hence 
^A = 
yxA 2 — yCt'A ' 2 
= A 2 -A' 2_ 
The following are the results of the experiments on glass :— 
3 L 
MDCCCLXXIX. 
