Measurements of Wave-Lengths. t)^y 
Figuring by Hydrofluoric Acid. 
It would be poor economy to employ any but the best 
surfaces in measuring work needing high accuracy; but there 
are occasions when all that is needed can be attained by 
more ordinary means. Common plate-glass is rarely good 
enough*; but I have found that it can be re-figured with 
hydrofluoric acid so as to serve fairly well, and the process 
is one of some interest. From what has been said already 
it will be understood that it is not necessary to make both 
surfaces plane, but merely to fit them together, which can 
be effected by operations conducted upon one only. 
Pieces of selected plate-glass, about J inch thick and of a 
size suited to the interference-gauges, were roughly shaped 
by chipping. The best surfaces were superposed and the 
character of the Jit examined by soda-light. One glass being 
rotated upon the other, the most favourable relative azimuth 
was chosen ; and by means of suitable marks upon the 
edges the plates were always brought back to the chosen, 
position. 
The principles upon which the testing is conducted have 
been fully explained in a former paper f. In the present 
case the surfaces are so close to one another that no special 
precautions are required. With a little management the 
contact is so arranged that a moderate number of bands are 
visible. If the fit were perfect, or rather if the surfaces 
were capable of being brought into contact throughout, these 
bands would be straight, parallel and equi-distant. Any 
departure from this condition is an error which it is pro- 
posed to correct. The sign of the error can be determined 
without moving the glasses by observing the effect of 
diminishing the obliquity of reflexion, which increases the 
retardation. Thus if a band is curved, and the change in 
question causes the band to move with convexity forwards. 
it is a sign that material needs to be removed from the parts 
of the glass occupied by the ends of the band. Such an 
operation will tend to straighten the band. If, however, 
the movement take place with concavity forward, then 
material needs to be removed from the middle parts. In 
every case the rule is that by removal of glass the bands, or 
any parts of them, can be caused to move in the same 
* If the surfaces are so shaped that the interference-bauds presented 
on superposition are hyperbolic, much may be gained by limiting- the 
aperture to a narrow slit corresponding to one of the asymptotes, espe- 
cially if the magnification used is in one direction only. 
f " Interference Bands and their Applications," l Nature,' xlviii. 
p. 212 (1893) ; Scientific Papers, iv. p. 54. 
