THE AID OF THE ACHROMATIC FRINGES. 



93 



through each other when the diaphragms vibrate. This shows itself in a 

 peculiar manner in the field of the vibration telescope, as indicated in figure 

 68. The whitish field carrying the fringe- waves a a, due to the fore-and-aft 

 motion of the mirrors m,m' (fig. 66) on the diaphragms of the telephones, 

 is intersected by nearly equidistant vivid lines b,b, normal to the waves. 

 The latter are apt to be broken and appear only as traces between the ver- 

 tical lines. The wave-length of a, a and the distance apart of b,b will depend 

 on the maximum speed of the objective of the vibration telescope, but the 

 distance bb and the form of aa usually have some simple relation to each 

 other, so that frequently the wave form is lost entirely and merely oblique 

 lines with the same inclination are seen between the lines b,b. 



To account for these occurrences of the lines b, it is sufficient to recall that 

 the originally coincident identical slit-images are in vibration through each 

 other in some direction relative to the lengths of the slits, effectively therefore 

 normal to this direction. The amount of this displacement is small, but it 

 is greater than the breadth of the fine slit-images. Hence these images will 

 be seen clearly only at the elongations when the slit-images are tempora- 

 rily stationary and at a maximum distance apart. These apparitions are 

 the lines bb and they belong to a system with higher frequency than the 

 objective. Hence, also, the waves a, a (fig. 68) are absent at b,b; for here 

 the slit-images are so far apart as to eliminate the interferences. 



It made little difference how the two telephones were connected. In fact, 

 one telephone may be thrown out of circuit. Vibration is thus communi- 

 cated mechanically similarly to the case when the fingers drum lightly on 

 the table. If a fine wire is drawn across the slit, the shadow remains straight 

 unless the vibrations are very intense; then beating waves in trains run 

 along the black line of shadow. 



The telephone diaphragms were now removed and replaced by the two 

 long strips of steel made from hack-saw blades 20 cm. long and about i cm. 

 broad and 0.06 cm. thick, rigidly attached to the body of the telephone by the 

 arch cd (fig. 69.) The mirror m was cemented to the middle of this strip, 

 balanced by a piece of iron on 

 the other side. To approach this 

 as near the magnet as possible, 

 forcing screws, e and/, were pro- 

 vided at a little distance from 

 the end of the strip. In this case 

 the vibration of the strip a b and 

 the mirror m at its middle was 



"-^===3==^ \M \\ 



fore and aft only, and as a con- / / f, i / "vsffn 



sequence the lines b, b' in figure 

 69 vanished completely. Here also the arrangement of telephones, whether 

 in series or parallel, made a decided difference in the amplitude of the 

 waves a, which could be increased many times the breadth between succes- 

 sive fringes before the waves became turbulent and broke up. 



