.Further Measurements of Wave-Lengths. 549* 
quite such close accord with the earlier ones as had been 
expected. Both sets are given below for comparison. 
Wave-lengths of Helium. 
II. 
III. 
7065-192 
7065-200 
6678-147 
6678-150 
5875-618 
5875-625 
5015-682 
5015-680 
4921-927 
4921-930 
(4713-173) 
4713-144 
(4471-480) 
4471*482 
The two last entries under II., enclosed in parentheses,, 
were obtained with the 1 mm. apparatus, and could not be 
expected to be very accurate. Preference may be given to 
III. throughout. 
These measurements of wave-lengths were not further 
pursued, partly because it was understood that other ob- 
servers were in the field and partly because my own vision, 
though not bad, is less good than it was. In particular at 
the blue end of the spectrum I found difficulty. It is evident 
that work of this sort should be undertaken only under the 
best conditions. 
One of the less agreeable features of the method is the 
complication which arises from the optical distance between 
the surfaces being slightly variable with the colour. In the 
earlier observations with a 5 mm. apparatus I was surprised 
to find the change amounting to 2\ parts per million between 
cadmium red and cadmium green. In the light of subsequent 
experience I am disposed to think that the silver surfaces 
must have been slightly tarnished. At any rate in the later 
measurements I found the difference much less, indeed scarcely 
measurable. It will be understood that no final uncertainty in 
the ratio of wave-lengths arises from this cause. Whatever 
the change may prove to be, it can be allowed for. 
Thirty Millimetre Apparatus. 
In this instrument the object was to construct a gauge with 
a much greater distance than usual between the plates, but 
otherwise on the same general plan as that of Fabry and 
Perot. The distance-piece A A, fig. 1, consisted of a 30 milli- 
metre length of glass tubing, each end being provided with 
three protuberances, equally spaced round the circumference, 
