THE AID OF THE ACHROMATIC FRINGES. 95 



proach the telephone magnets tt' as near the soft-iron armatures hh' as 

 possible without overstepping the unstable position, in view of the tension 

 of the tense wire e, e'. To prevent the sticking of h to t, which is very annoy- 

 ing, small rubber buffers may be placed between. It is not usually practi- 

 cable t approach h to t by more than i or 2 mm. and keep h perfectly free. 

 To give the vibrating system adequate damping, thin wires qq', less than a 

 millimeter thick, bent, and dipping into lubricating oil in small vats p,p', 

 suffice. To change the damping the latter may be lowered or even removed. 

 The fibers e,e' were about 45 cm. long and their distance apart about 29 cm. 

 Their period and that of the vibrating telescope were made about the same, 

 on the average about 0.2 sec., and this was for convenience nearly the same 

 .as the period of the vibrating telescope and of the induced alternating current. 



It is convenient to insert an extra telephone (resistance about 100 ohms) 

 in circuit, in order to insure against breaks of contact or other discrepancy, 

 when the perturbation of fringes ceases. 



As a generator an earth inductor with a coil of wire 60 cm. in diameter 

 was at first used. It was turned by a small motor, and by putting a sliding 

 rheostat in circuit the period could be varied from about 0.19 to 0.26 second. 

 To measure the average intensity of current a Siemens precision dynamometer 

 was installed ; but though indicating currents as low as even within one-tenth 

 of an average milliampere, it was not influenced by the earth inductor, though 

 at maximum speed. This was therefore replaced by a small magneto con- 

 sisting of a bar-magnet about 6 inches long, rotating in an oblong coil. By aid 

 of the rheostat and appropriate pulley-wheels large differences of speed could 

 be obtained and maintained at any value, so that periods from about o.i 

 to 0.3 sec. were available. With a period of 0.2 sec., moreover, it showed 

 a deflection on the dynamometer and, being in general lighter and more 

 easily controlled, was preferable to the earlier instruments. 



The three vibrating systems (mirror, telescope, alternator) thus all admit 

 of an adjustment of their periods, and these should be nearly the same if 

 the elliptic system of Lissajous curves are to be obtained, which is the prefer- 

 able case. A change of the tension of the wires ee' in figures 70 and 71, or 

 any adjustments at the telephones, calls for a fresh search for fringes; but 

 this is not difficult if the spectro-telescope is first used and the admonitions 

 relative to the objective coincidence of pencils entering the telescope, as well 

 as their parallelism, as above explained, are given consideration. These 

 difficulties do not enter when the mirrors can be displaced normally to the 

 incident rays. 



In addition to the telephone it' in figure 71, coils in great variety were 

 used. The telephones were also placed within and without the rectangle of 

 wires e,e' and in the same or on opposite (as in fig. 71) sides of mm'. But 

 the phenomena in such cases were not dissimilar nor advantageous. 



For general purposes the mass of the vibrating system mm' should be 

 diminished, as it could easily be ; but the straight blade of glass (in preference 

 to two small mirrors) is a convenience in adjustment and here suffices. 



