THE AID OF THE ACHROMATIC FRINGES. 



77 





54 



glass about 32 cm. broad and 45 cm. high, spaced by two strips if, 3 cm. 

 wide, of thick (9 mm.) plate glass, reaching not quite from top to bottom. 

 Six steel clips h,h,h,k,k,k, (such as are used for binding pamphlets) held 

 these strips in place and also clasped securely on the outside two wooden 

 strips of about the same width and one-half inch thick. At the top of the 

 wooden strips, two nipples, s and t, of >^-inch gas-pipe, projecting normally 

 to the strips, served for the hanging of the case and needle from a firm wall- 

 bracket. It would have been preferable to use the wood strips (without the 

 glass strips) clasped between the glass 

 plates A, A' as spacers and hangers at 

 once, and this was eventually done. The 

 bottom of the case is subsequently to be 

 closed from below by a strip of felting gg. 

 To diminish the space within the case 

 two thin cloth-covered wooden boards 

 w,w' are inserted from the top. The 

 quartz fiber q, to carry the needle mm, 

 hangs from a long Y 8 -inch brass rod d, 

 which may be raised or lowered in view 

 of the sleeve e, held by a separate ad- 

 justable arm without. The rod d must 

 fit the perforation in the cork nicely, so 

 that the former may be smoothly raised 

 or lowered and held in any position by 

 virtue of friction. To swing the needle this is first placed on cork Y's below 

 the opening of the case A, A', the felting gg having been removed. The 

 quartz fiber is then lowered on the long stem d, until the lower hook on 

 the fiber is in position to grasp the clasp on the needle mm, still below the 

 case. The needle is then cautiously lifted by raising d until it has the 

 required position relative to the interferometer, about as shown in the figure. 

 After this the felt strip gg is inserted to close the case, and the necessary 

 adjustment made at e and d to swing the needle freely in the restricted space 

 provided for it. 



Observations were made with the needle at some length. The quartz 

 fiber used was L=iy cm. long; the distance d apart of attracting weights 

 M and attracted weights m was 6 cm. Hence for M= 10 gr. 



^ 



7 = 



23Xio 3 X75 2X17 10 



Thus, if r= 10 2 cm. 

 r= io~ 3 cm. 



AA/"=i.iXio 6 cm. per kg. of M 7 = 4.6 sec., 

 AAT=i.iXio~ 2 cm. per kg. of M T = 465 sec. 



The first case would then correspond to but one-fortieth of a fringe; in 

 the second case there should be 250 fringes per attracting kilogram. 



If the tenacity of quartz be taken as i.sXio 3 kg. per sq. cm. the latter 



