[ 548 ] 
LI. Further Measurements of Wave-lengths, and Miscel- 
laneous Notes on Fabry and Perot's Apparatus. By 
Lord Ratleigh, O.M., P.R.S.* 
IN a former paper f I described a modified form of apparatus 
and gave the results of some measurements of wave- 
lengths, partly in confirmation of numbers already put 
forward by Fabry and Perot and partly novel, relating to 
helium. I propose now to record briefly some further 
measures by the same method, together with certain obser- 
vations and calculations relating thereto of general optical 
interest. 
The apparatus was arranged as before, the only change 
being in the interference-gauge itself. The distance-pieces, 
by which the glasses are kept apart, were now of invar, with 
the object of diminishing the dependence upon temperature. 
The use of invar for this purpose was suggested by Fabry 
and Perot, but I do not know whether it has actually been 
employed before. The alloy was in the form of nearly 
spherical balls, 5 mm. in diameter, provided with projecting 
tongues by which they were firmly fitted to the iron frame. 
The springs, holding the glasses up to the distance-pieces, 
were of the usual pattern. The whole mounting was con- 
structed by Mr. Enock, and it answered its purpose 
satisfactorily. There is no doubt, I think, as to the advantage 
accruing from the use of invar. 
The measurements were conducted as explained in the 
earlier paper. The first set related to zinc which was com- 
pared with cadmium. Both metals were used in vacuum- 
tubes, of the pattern already described, with electrodes merely 
cemented in. It was rather to my surprise that I found 
ordinary soft glass available in the case of zinc, but no diffi- 
culty was experienced. The former observations with the 
" trembler " suggested a wave-length for zinc red about 
one-millionth part greater than that (6362*345) given by 
Fabry and Perot. This correction has been confirmed, and I 
would propose 6362*350, as referred to Michelson's value of 
the cadmium red, viz. 6438*4722. No difficulty was expe- 
rienced in identifying the order of the rings by the method 
formerly described and dependent upon observations with 
the gauge alone. 
The results of the measurements upon helium were not in 
* Communicated by the Author. 
t Phil. Mag. [6] vol. xi. p. 6S5 (May 1906). 
