370 
MESSRS. W. E. WILS077 AXD P. L. GRAY 
insulated) to a fine g'lass tube, G.T., to wliicli is fastened tbe mirror, J/. The quartz- 
fibre suspension, Q.F., is held by the pin, P, which passes through a cork, as shown in 
Eig. 4. 
The Differential Radio-inici’ometer and Circuit. 
the quarter-scale drawing. The weight of the entire system below the pin is about 
H gi’ain. 
In the elevation of the complete instrument, Mag. denotes the magnet, N and S 
the pole pieces, between which the circuit hangs inside a hollow block of brass, 
with an iron core as in the ordinary form of the radio-micrometer. Z is a lens, which, 
with the small mirror, AI, forms an image of a luminous slit, on a scale at a distance 
of about a metre. 
IF../, and W'.J'. are water-jackets, through which it was found better not to allow 
the water to circulate. They were kept filled, however, to prevent sudden changes 
of temperature from affecting the circuits. 
The lower water-jacket rests upon a disc of mahogany, which is supported by a 
brass pillar; the details of the remaining parts of the instrument will be obvious 
on an inspection of the diagram. 
The water-jackets are pierced by tubes, through which the receiving surfaces are 
