MR. A. B. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 357 



parallel, but that of the cover-wedge above and that of the counteracting wedge 

 below. 



The adjustment of the upper surface of the aluminium, and of the lower surface of 

 the cover-wedge, was carried out precisely as has been described for the tripod table 

 and cover-wedge ; the two parts of the apparatus were arranged at close quarters, 

 and the images of the signal-stop obtained in white light with the aid of the single 

 reflecting prism were brought to the desirable partially overlapping position, and 

 isolated by the iris diaphragm from all other radiations. The single prism was then 

 exchanged for the train of refracting prisms, the white light was replaced by red 

 hydrogen light from the Geissler tube, and the simple eyepiece was replaced by the 

 micrometer combination, when very brilliant bands were at once observed. After 

 adjustment of the width of the bands and their parallelism to the vertical spider- 

 lines, the platinum-indium screws were fixed, the bent thermometer was arranged 

 in position, and the expansion apparatus removed to its proper distant position. The 

 interference tube was then raised, while the bath, containing its dish of vitriol, was 

 arranged in position; the interference apparatus was then lowered into the bath, 

 the thermometers fixed, and the whole left overnight to attain equilibrium of 

 temperature. 



Next morning the vitriol was removed, and the inner thermometer read while the 

 interference apparatus was raised for this purpose ; the barometric pressure was also 

 taken, the adjustment of the bands for height effected, the silver ring being brought 

 into contact at its lower limb with the horizontal spider-line, and the measurements 

 of the positions of the centre of the silver ring and of the five adjacent bands were 

 carried out in red hydrogen light. This light is particularly suitable, on account of 

 the bands being separated at a greater distance than with the other available wave- 

 lengths, and also by reason of the particular brilliancy of this radiation afforded by 

 the Biedel- Geissler tube at the ordinary temperature of the tube. The better of the 

 two parallel surfaces of the aluminium block, which was placed uppermost for use, 

 afforded bands which were strictly regular and almost perfectly rectilinear, yielding 

 excellent measurements. The temperature was then raised to the neighbourhood of 

 70, recording the transit of bands on the tape, and after a couple of hours' constancy 

 measurements were made at this temperature. After the attainment of another 

 interval, the passage of bands being recorded as before, a final set of measurements 

 were made for a constant temperature in the neighbourhood of 120. 



After cooling overnight, a duplicate series of determinations for three similar 

 temperatures were made next day. 



Lastly, after cooling during another night, the exact lengths of the platinum- 

 iridium screws projecting above the height of the points were measured by the aid of 

 the thickness measurer ; similarly, the known thickness of the aluminium block was 

 verified. 



