210 Mr. A. E. Tutton. 



then pass through the large cover-disc laid on the tripod screws, 

 the under surface of which is one of the two surfaces the reflections 

 from which are to be made to interfere ; the other of the two relevant 

 surfaces is the upper surface of the crystal supported on the com- 

 pensator, or in the case of a badly reflecting crystal, of a small disc 

 of black glass, polished above and ground below, which the author 

 lays upon it. The large cover-disc and the glass roof-disc are slightly 

 wedge-shaped to the extent of 35', and are arranged complementarity 

 so as to counteract the dispersion produced by each other ; by using a 

 wedge-shaped cover-disc the undesired reflection from the upper 

 surface can be deflected out of the field of the telescope, and the 

 tilt of the roof-wedge is given for a similar reason. After reflec- 

 tion from the two surfaces relevant to the interference, the rays 

 re-traverse their path, but instead of doing so absolutely are made 

 to pass through the clear half of the telescope diaphragm to the 

 observing lens arrangement. An iris diaphragm is placed against 

 the main diaphragm to assist in further excluding undesirable 

 radiations, and the illuminated surface of the small reflecting prism 

 can be more or less curtailed by suitable rectangular signal-stops. 



There are two observing lens arrangements : one a simple eye- 

 piece and the other a micrometer combination of three lenses. The 

 first enables the observer to properly adjust the images of the signal 

 stop in white light reflected from the two relevant surfaces of the 

 interference apparatus, so as to cover each other to the extent 

 required to produce interference bands of requisite width. The 

 micrometer combination converts the observing apparatus into a 

 microscope wherewith to view the bands. The spider-lines of the 

 micrometer are simultaneously visible. The reference point of the 

 interference apparatus is the centre of a minute silver ring on the 

 under surface of the cover-wedge, and the two vertical spider-lines 

 can be adjusted by a special drum to such a separation as enables the 

 inner circle of the ring to be brought symmetrical to them r showing 

 equal suitable arcs outside each ; this separation is also convenient 

 for the width of band generally employed, which corresponds to 

 100 drum divisions of the other drum which moves both spider-lines 

 simultaneously. 



The interference chamber is provided with an adjusting table of 

 non-conducting material, and is quite open to the heated air of the 

 bath, being provided with large windows, which are also very useful 

 for the adjustment of the tripod. The heating bath is a double 

 air bath of copper suitably screened in every direction by asbestos 

 millboard. It is provided with a thermostat in the outer bath and 

 two thermometers in the inner bath. The expansion apparatus is 

 immersed in the latter up to a third of the porcelain tube. The 

 actual temperature of the tripod is ascertained by a third thermo- 



