A Compensated Interference Dilatometer. 209 



The instrument consists of two independently mounted portions, 

 which the author terms respectively the expansion apparatus and the 

 illuminating and observing apparatus. For the purposes of pre- 

 liminary adjustment the two portions are arranged at close quarters, 

 while during the observations they stand at opposite ends of the slate 

 table, 6 feet in length, upon which they are mounted, the expansion 

 apparatus resting on a separate and movable cloth-lined slab for the 

 purpose of effecting the transfer from one position to the other. 

 Hence the optical measuring apparatus is far removed during obser- 

 vations from the heated atmosphere of the chamber containing the 

 tripod. 



The illuminating and observing apparatus consists of a telescope 

 arranged with a side tube for auto-collimation. It is mounted on a 

 stout pedestal provided with three legs and levelling screws, and its 

 height can be varied by the vertical rack and pinion movement of a 

 stout inner column. At the common focus of the lens of the side tube 

 and of the telescope objective a small totally-reflecting prism is 

 placed, half closing the aperture of a diaphragm placed on the 

 objective side of the prism in the main optical tube ; the prism is so 

 arranged that the light from the illuminating lens is reflected 

 through the diaphragm to the objective. It passes thence, as parallel 

 rays, across the intervening space to the expansion apparatus, at the 

 summit of which it meets with one of two interchangeable deflecting 

 arrangements which direct the rays vertically down the tube of the 

 expansion apparatus into the interference chamber containing the 

 tripod. One of the two is a large totally-reflecting prism ; this is 

 used when white light or a sodium flame is the source of light 

 placed before the illuminating lens, the former for adjusting pur- 

 poses and the latter for generating the bands. The other is a train 

 of two refracting prisms whose total minimum deviation averages 

 90 ; this is used when a hydrogen and mercury Geissler tube is the 

 source of light; the dispersion being then adequate to effectively 

 separate the red C from the greenish-blue F radiations, or both from 

 the mercury green, radiation, when it is desired to generate bands in 

 C or F hydrogen light, or that corresponding to the green mercury 

 line as recommended by Pulfrich. 



The expansion apparatus is suspended from an arm carried by a 

 pedestal provided with rack and pinion vertical adjustment as in the 

 case of the observing apparatus. Below the deflecting prism or 

 prisms the short metal tube passes into a longer one of porcelain, 

 which at its lower end is fitted into a further short metal tube carry - 

 . ing below the interference chamber within which the tripod is 

 placed. On passing down the tube from the deflecting apparatus the 

 rays pass first through a slightly tilted, thick glass disc held in a 

 diaphragm and forming the roof of the interference chamber. They 



