INTO THE FORM OF THE WAVE-SURFACE OF QUARTZ. 
305 
thing was ready for observation. The temperature was taken, usually at the be¬ 
ginning and end of a set of observations, from a thermometer lying on the table by 
the side of the spectrometer. 
The appearance seen in the telescope was the needle-point standing out against 
an irregular patch of light crossed by one or more dark bands. The bright patch 
was really the aperture of the analysing Nicol, and as the telescope was moved 
round one or two degrees the patch moved across the needle-point. This was 
due to the light which reached the eye coming from a different part of the object- 
glass and thus passing in a different direction through the analyser.* Thus it was 
really equivalent to a slight rotation of the analyser round its axis. A similar effect 
was produced by turning the table owing to the lateral displacement of the beam of 
light by the quartz. To avoid the latter evil I at first always turned the telescope 
till the needle-point was exactly in the centre of the patch before I took a reading. 
This was done in most of the observations on Plate 1. But for the small rings in 
Plate 2 I simply clamped the telescope in the position where the Nicols were crossed, 
and kept it there. By trying exaggerated cases I found the error was very small 
and it tended to increase the radius on one side while diminishing it on the other. 
The rings of course were fixed relatively to the quartz, so that the difference 
of the readings of the table vernier for the two sides of the ring being on the 
needle-point gave the diameter of the ring in air. 
For the first half dozen rings it may be shown, that the mean of these two 
readings should coincide, or in other words that the rings are practically circular 
and have a common centre, provided that the deviation of the axis from the normal 
do not exceed two or three degrees. This common centre may be called the axis 
in ah’, and is related to the real optic axis by the ordinary law of refraction. From 
this it is easy to see how the deviation of the axis from the normal was determined 
from the measurements of the rings. I found that the error was 1° 45' measured 
parallel to plane A and 18' measured perpendicular to plane A. It is clear since 
the axis lay 18' out of plane A that instead of measuring the diameter of a ring 
I was only measuring a chord. The small correction for this was however easy 
to apply. 
But for the measurements at right angles to plane A this chord would have 
been perhaps only half the diameter for the first ring if I had proceeded as 
before. It was necessary to tilt the plate forwards till the axis in air came 
nearly into the plane of measurement. I was at a loss for some time how to 
produce this large tilt on the spectrometer with any accuracy. At last I Jut 
* When the direction of light incident on a Nicol’s prism is inclined to the principal plane of the 
prism at an angle a, then, in order that the light may be completely quenched by the prism, its plane of 
polarisation must be inclined at an angle a/3 nearly, to the plane through the ray most nearly parallel to 
the principal plane of the prism. See a paper by the author, Phil. Mag., vol. 19 (1885), p. 320.— 
[June 15, 1886.] 
MDCCCLXXXVI. 2 R 
