556 Lord Rayleigh : Further 
•direction as that in which they move when the obliquity of 
reflexion is diminished. 
In carrying out the correction, the plate on which it is 
intended to operate is placed below, and it is convenient if! 
it be held in some form of steady mounting so that the 
upper plate can be removed and replaced in the required 
position without trouble. The acid, two or three times 
diluted, is applied with a camel's hair brush and after being 
worked about for a few seconds is removed suddenly with a 
soft cloth. Endeavour should be made to keep the margin 
of the wetted region moving in order to obviate the forma- 
tion of hard lines. Success depends of course upon judge- 
ment and practice, and the only general advice that can be 
given is to make a great many bites at the cherry, and to 
keep a record of what is done each time by marking suitably 
on one of a system of circles drawn upon paper and repre- 
senting the surface operated on. After each application of 
acid the plates are re-examined by soda-light and the effect 
estimated. The difficulty is that in most cases the bands are 
not reproduced in the same form. In one presentation the 
error may reveal itself as a curvature of the bands and in 
another as an inequality in the spacing of bands fairly 
straight. Often by a little humouring the original form 
maybe approximately recovered, and in any case the general 
rule indicates what needs to be done. 
By this method I have prepared two pairs of plates which 
perform very fairly well, but of course only when placed in 
the proper relative position. The operations, though pro- 
longed, are not tedious, and I doubt not that with perse- 
verance better results than mine might be achieved. The 
surface of the glass under treatment suffers a little from the 
development of previously invisible scratches in the manner 
formerly explained, but the defect hardly shows itself in 
actual use. I have not ventured to apply the method to 
surfaces already very good such as those supplied by the 
best makers for use in Fabry and Perot's apparatus; but I 
should be tempted to do so if I came across a pair suffering 
from slight general concavity. The application of acid would 
then be at the outer parts. In the best glasses that I possess 
the error is one of convexity. 
Effect of Pressure in Fabry and Perot's Apparatus. 
The observation that the rings were more sensitive than 
had been expected to the pressure by which the plates are 
