THE AID OF THE ACHROMATIC FRINGES. 



75 



as 15 cm. long and 2.5 cm. in diameter, but to be changed in capacity with the 

 balls m,m' used. The stems of the hooks, screwed and sealed into the tube 

 rr, carry the mirrors n, n' . The needle is suspended from a torsion-head 

 by the bifilar of silk fiber kl, also com- 

 pletely submerged in the water-bath. 

 The hooks k and I are provided with a 

 thin flat sheet-metal link, by which the 

 bifilar may be appropriately spaced. 



The needle system is so adjusted as 

 just to float. It is then weighted by 

 i or 2 grams to sink it. The weight in 

 water may be measured at K. In view 

 of the weight of needle, the mirrors n,n' 

 may be rigidly connected by a very nar- 

 row strip of this plate glass, which facili- 

 tates adjustment. 



The attracting weights M and M' up 

 to 5 kg. were used in pairs M,Mi and 

 M'.M'i, on each side suspended by a 

 steel belt from a pulley overhead in such 

 a way that when MM' are i n P lace M'iMi are raised out of effective reach. 



The chief difficulty was encountered in floating the needle. When this 

 was done the whole tank was fastened in place on the interferometer, as the 

 torsion-head at k is attached adjustably to the tank. 



The fringes were found without much difficulty ; but they were in incessant 

 motion, owing no doubt to eddy currents produced by temperature differ- 

 ences. After many trials I concluded that measurements would be untrust- 

 worthy and further trials were therefore abandoned. The experiment is 

 in fact too difficult for a single observer and would be feasible only in an 

 environment of perfect quiet and constant temperature. 



43. Expeditious fringe detection. In work like the present it is necessary 

 to find the fringes quickly under considerable disadjustment of parts. In 

 this case, if the auxiliary mirror m (fig. 4, Chapter I) can be manipulated, 

 the two images in the telescope T are first made coincident by adjusting 

 M', in which case the rays are parallel as they enter T. The mirror m is 

 then rotated around the vertical and the horizontal axis, until, to the eye, 

 the spots of light coincide locally on the face of M'. Fringes when found by 

 moving the micrometer here are then strong. If. mm can not be interfered 

 with, the rays are first made parallel as before by the coincidence of images 

 in T. Thereafter the mirrors M and M' are rotated around a horizontal 

 and a vertical axis in parallel (i. e., successively or alternately retaining their 

 parallelism) until the spots of light on M' again coincide, locally, to the eye, 

 or better, when caught objectively on a screen. It is clear from figure 2, 

 Chapter I, that the remoter ray from M is displaced on the screen more 



