340 MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



of somewhat over a centimetre. At this distance it is most difficult to generate 

 satisfactory bands with C or F hydrogen light, and it is only at all possible with 

 sodium light provided the exact distance of a FIZEAU maximum is attained. The use 

 of the light corresponding to the green mercury line, as recommended by PULFRICH, 

 affords, fortunately however, excellent bands in the author's apparatus, even at 

 distances considerably beyond 2 centims. At 12 millims., between glass 

 surfaces, the bands are really admirable. A further difficulty, however, arose, 

 owing to the greater volume of reflection from the polished plane surface of the 

 platinum-iridium table than from the glass cover-wedge, the excess of light from the 

 former illuminating the dark spaces and causing the bands to appear very faint. 

 This difficulty was overcome by depositing upon the relevant glass surface, by means 

 of a milk-sugar ammoniacal silver solution, a thin film of silver of the right density 

 to afford the necessary increased reflection from the glass and diminished reflection 

 from the platinum-iridium. After numerous attempts a film was obtained winch 

 caused the reflections to be almost exactly equal, and consequently enabled excellent 

 bands to be obtained. The reference mark of the disc was a minute central ring, 

 where the silver had been scraped off by a needle point whilst rotating the disc on a 

 microscopist's turn-table. It should be remarked that this cover-wedge, employed 

 for the purpose of the determination of the expansion of the tripod, was a duplicate 

 of the ordinary one carrying the silver ring previously referred to. A corresponding 

 duplicate smaller disc was employed in connection with it to close the upper aperture 

 of the interference chamber, both being cut from the same slightly wedge-shaped 

 slab of glass, as in the case of the ordinary pair. The two pairs, being of slightly 

 different angle, were provided with distinguishing marks on their edges to prevent 

 bhe use of a wrong combination. 



The screws of the tripod were adjusted so that about 12 millims. length projected 

 oeyond that side of the table which carried no points, and the tripod was placed in 

 the interference chamber with this side uppermost. The glass floor of the chamber had 

 previously been made exactly horizontal with the aid of a circular spirit level and the 

 adjusting screws. In order to prevent reflection of light from the marginal portion 

 of the platinum-iridium surface, a lens atop of 15 millims. aperture and convenient 

 diameter was laid upon it as a mask. The cover-wedge was then laid upon the 

 screws with the silvered side downwards, and the engraved chord, marking the 

 direction of the wedge, arranged perpendicular to an imaginary line at this height 

 ioining the pedestals of the expansion and the observing apparatus, that is, running 

 right and left of the observer in front of the latter. The counteracting upper wedge 

 was arranged with its chord in a parallel direction and on the same side of the centre 

 to that of the cover-wedge, but below, while that of the cover-wedge was uppermost, 

 on the non-silvered side. 



The expansion apparatus was arranged close up to the observing apparatus for the 

 adjustment, and carried the large single reflecting prism in position at its summit. 



