INTO THE FORM OF THE WAVE-SURFACE OF QUARTZ. 
307 
impaired the accuracy of measurement except perhaps slightly in the first two rings. 
I had the plate cut down to this thickness in order to secure good observations at the 
high angles of incidence. The object glasses of the telescope and collimator were 
barely 25 mm. in diameter, yet I was able to obtain good readings for the 150th ring 
at an angle of incidence of over 74°, which would have been quite impossible with a 
plate 25 mm. thick. 
The observations on the small rings in Plate 2 were similar to those in Plate 1, 
except that three readings were taken throughout for each side of the first ring and 
two readings for each side of the second ring. The greatest difference between the 
readings was 1^' in the first ring and l' in the second. As will be seen below the 
first ring gave results which were at first sight rather discordant with the others. So 
I made a few special observations on the first ring with a very bright sodium-flame. 
This was obtained, without the use of oxygen, with the aid of a thin platinum wire 
carrying a very small bead of carbonate of sodium. The thin wire was raised to an 
intense heat in the hottest part of the flame, while sufficient liquid oozed down the 
wire from the molten bead to replace the sodium given off. The flame was nearly if 
not quite as bright as if oxygen had been blown in, and the definition of the first ring 
was decidediy improved ; though traces of other colours appeared "which were a little 
troublesome. 
The measurements of the larger rings deserve a few remarks. The rings were 
narrow and sharply defined, so the needle-point, could be placed with great accuracy, 
and another important source of error was removed, for turning the polariser or 
analyser slightly would not appreciably displace the ring. I believe the discrepancies 
between different measurements that appear in the tables arose chiefly from errors of 
in reading the verniers. Owing to the lateral displacement of the light when the 
plate was placed obliquely, the telescope had to be moved various distances up to 
about 1° to either side of its mean position. To make allowance for this motion the 
telescope vernier had to be read. Thus each diameter given depends on four vernier 
readings. It is not then very surprising that two diameters should occasionally differ 
by 1-^'; while the mean of the four diameters is probably very accurate. 
The patch of light alluded to previously as forming the field of view was, as the 
angle of incidence increased, gradually encroached on by the edge of the object-glass 
of the telescope. This was avoided by moving the telescope. But soon the edges of 
the quartz plate began to encroach, and this was unavoidable. When the 150tli ring 
was on the needle-point the field of view was very narrow, so the readings were not 
quite as accurate as for the other rings. Still I do not think the error of setting the 
needle-point was ever as much as 1'. Notwithstanding the movement of the tele¬ 
scope the light at the high angles of incidence entered near the edge of the object- 
glass ; so I carefully tested the telescope for spherical aberration and found that no 
appreciable error could arise from this cause. As a matter of curiosity 1 found it was 
possible to obtain a reading of both sides of the 163rd ring. 
2 r 2 
