Measurements of Wave-Lengths, 553 
side of the arcs. The perpendicular direction of vision may 
be tested by observing the reflexion of the eye itself in the 
silvered surface. 
Behaviour of Vacuum- Tabes. 
The form of vacuum-tube described in the first paper, and 
depending on sealing-wax for air-tightness, continues to give 
satisfaction. As already mentioned, though made of soft 
glass, they are available for zinc, and the cadmium tubes have 
lasted well with occasional re-exhaustion. It is advisable to 
submit them to this operation when the red light begins to 
fall off. After one or two re-exhaustions the condition seems 
to be more durable. 
With thallium my experience has been rather remarkable. 
The green light is very brilliant and offers a further advantage 
as being comparatively free from admixture with other 
colours *. But Fabry and Perot found thallium tubes to be 
very short-lived, sometimes lasting only a few minutes. I have 
used but one thallium tube, of the same construction as the 
others, and charged with a little thallium chloride. This tube 
has been used without special care on many occasions — I cannot 
say how many, but probably seven or eight times, — and it does 
not appear to have deteriorated at all. It looks as though 
the chloride had decomposed and metal had deposited upon 
the aluminium electrodes. But what the circumstances can 
be that render my experience so much more favourable I am 
at a loss to conjecture. 
The same form of tube answers well for mercury, but 
with this metal there is usually no difficulty. 
Control of the figure of the glasses by bending. 
Very good plates can now be procured from the best 
makers, but on careful testing they usually show some defi- 
ciency, mostly of the nature of a slight general curvature. 
Thus when in Fabry and Perot's apparatus the adjustment 
for parallelism is made as perfect as possible, the rings may 
be observed to dilate a little as the eye moves outwards in any 
direction from the centre towards the circumference of the 
plates. This indicates a general convexity. 
It occurred to me that an error of this kind might be 
approximately corrected by the application of bending forces 
to one of the plates — it does not matter which. The easiest 
way to carry out the idea is to modify the apparatus in such a 
way that the points of application of external pressure are 
* The green line is known to be itself double. 
