PHYSICS: C. BARUS 
333 
vibration telescope, V. The secondary S was the coil of the pre- 
ceding paper, wound on glass. This was in circuit with a rheostat R 
(up to 40000 ohms), the telephones T, T , T", and the key K. The 
condenser C in the secondary is available when needed, and the in- 
ductive resistance e" in the primary. 
The whole of the parts shown in figure 1 could be slid fore and aft on 
the carriage BB and AA\ to accommodate the interferometer. 
3. Observations with the Slit Image— It will be seen that if an ordi- 
nary telescope is used at V , figure 2, the sharp slit image must widen 
to a band here, as in the preceding apparatus; but the design is much 
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less sensitive because the rays are parallel throughout. The slit image 
nevertheless suffices admirably for finding the resonance tension. For 
this purpose the screw h or h' , figure 1, is first manipulated till the 
image begins to widen. The fine adjustment is then made at w, x, 
till the maximum band- width is reached. One easily recognizes in this 
way three harmonics, the fundamental at lowest tension and small 
band- widths; the octave at larger tension with maximum band- width; 
and the next overtone at still larger tension and diminished band- width. 
Above this I did not go as the stress on the wire would have been 
excessive. The reason for the prominence of the octave here, is not 
