550 Further Measurements of Wave-Lengths* 
at which the actual contacts took place. The removal of the 
intervening material and the shaping of the protuberances 
were effected with a file moistened with turpentine. 
Against this distance-piece the glass plates B B are held 
by the arrangement shown in fig. 1. The lower plate B 
rests upon a brass ring C to which the brass castings D are 
rigidly attached. The upper ring E is connected with the 
castings only through the steel springs F. Both rings are 
provided with protuberances in line with those on the glass 
cylinder, and the pressure is regulated by the screws Gr. The 
whole was constructed by Mr. Enock. Some little care is 
required in putting the parts together to avoid scratching the 
half-silvered faces ; but when once the apparatus is set up its 
manipulation is as easy as that of the ordinary type. 
In all interference-gauges it is desirable that the distance- 
pieces be adjusted as accurately as possible. For although 
a considerable deficiency in this respect may be compensated 
by regulating the pressures (see below), the adjustment thus 
arrived at is less durable, at least in my experience. Even 
when the distance-pieces are themselves well adjusted, it is 
advisable to employ only moderate pressures. 
Observations with the 30 mm. gauge have been made upon 
helium, thallium, cadmium, and mercury. In the first case 
the (yellow) rings are faint, the retardation being not far 
from the limit. Indeed when at first it was attempted to 
adjust the plates with helium, the rings could not be found. 
With thallium also the rings were rather faint, but with 
mercury and cadmium there was no difficulty. 
Magnifying Power. 
At a distance of 30 mm. the rings are rather small, and 
one is tempted to increase the magnifying power of the 
observing telescope. As to this there should be no difficulty 
if the aperture could be correspondingly increased. But 
although the plates themselves may be large enough, an 
excessive strain may thus be thrown upon the accuracy of the 
figuring and upon the adjustment to parallelism. If, on the 
other hand, the aperture be not increased, the illumination of 
the image falls and the extra magnifying may do more harm 
than good. 
A means of escape from this dilemma is to effect the 
additional magnification in one direction only, which in the 
present case answers all purposes. When straight inter- 
ference-bands, or spectrum lines, are under observation, 
there is no objection to astigmatism, and we may merely 
