A XEW INTP]RFEKEXTIAL FORM OF ELASTICFrY ARPARA^rUS. 
145 
chamber carried at its upper eud a tliick glass disc, which, being equally slightly 
wedge-shaped (35 minutes) and arranged in the inverse way, corrected for the slight 
dispersion of the large glass disc of the interference apparatus, a milled cap is 
provided to contain this correcting disc. The new cap screws on to the lower 
extremity of the suspended tube; it has the same adequate clear aperture as the 
removed interference chamlier, and is similarly provided with two raised points in 
its rabbett to tilt the disc to the minute extent recpiired to throw its reflections out 
of the way of the interfering rays from the interference apparatus. 
I'he. Elasticity A2')paratns. 
The new apparatus consists of three distinct portions : (1) the apparatus winch * 
carries (a) the knife-edges, against which the plate of the substance under investi¬ 
gation is to l)e bent, (6) tlie transmitter of its bending movemeiit, (c) the interference 
tripod, and (d) a pair of mechanical “ fingers ” for adjusting the plate ; (2) a delicate 
balance, through a point at one end of the beam of which the bending weiglit is 
apjflied, and a device for controlling its application ; (3) a measuring microscope, 
reading in two rectangular directions to a thousandth of a millimetre. 
The whole of these parts, together with the pedestal of the suspended optical tiflje 
VOL. ceil.— A. 
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