144 
DE. A. E. H. TITTTON ON THE ELASMOMETEE, 
The Interferometer. 
The observing telescope, with its auto-collimator and attached hydrogen Geissler 
tube, shown in fig. 1 standing on the adjustable pedestal in the foreground, is 
precisely as described in the dilatometer memoir {he. cit., p. 324). 
The similar pedestal in the Ijackground carries the train of two reflecting prisms ; 
these are arranged to direct the particular rays to be employed, those corresponding to 
the red C-hydrogen line, down the vertically suspended tube, of which the greater 
Fig. 1. 
portion is of Berlin porcelain, on to the interference apparatus. With the exception 
of the counterpoise on the backward prolongation of the arm from Avhich the tube 
is suspended, which exj^erience since the publication of the dilatometer memoir has 
shown to be a valuable addition, this portion of the apparatus is also exactlv as 
described in tliat memoir (p. 321) as far as the lower end of the porcelain tube and its 
short gun-metal continuation. 
Fi'om the end ol the latter the interference chamber of the dilatometer has 
been removed. The interference tripod is carried by the iieAv apparatus itself, 
immediately below the end ol the vertical tube. As the removed interference 
