468 MR. A. E. TUTTON ON THE THERMAL DEFORMATION OF THE CRYSTALLISED 
dilatometer {loc. cit., lower part of p. 363 and p. 364), after the description and com¬ 
munication of the results of the determinations of the expansion of the platinum- 
iridium alloy of the interference tripod and of the aluminium of the compensators. 
The temperatures employed have not been quite so high as in the cases of those 
metallic substances, the highest limit being in the neighbourhood of 96°, in order 
that there might be no appreciable deformation due to internal strain, provoked by 
the attempted vaporisation of the water of mother-liquor contained in the inevitable 
minute internal cavities. It is impossible to altogether prevent the formation of such 
cavities, even by slow evaporation in vacuo, but the remarkable agreement of the 
results obtained indicates that any variable deformation due to this cause has been 
infinitesimal. 
Every efibrt has been made to render the conditions of the determinations as 
rigidly analogous as possible, so that comparisons of the results can be made with 
confidence. As far as possible the same aluminium compensator has been used 
throughout, namely, one 5'25 millims. thick and a centimetre diameter, and un¬ 
provided with points as the three-point method of contact was so rarely available; 
where exceptions have been made, results with the compensator mentioned are avail¬ 
able for the same direction of the same salt, and the two series of results agree so 
well that the change has evidently not introduced any error. This, of course, should 
be so, for the compensators, including the one 12 millims. thick used for the deter¬ 
mination of the expansion of the metal, were all cut from the same casting of pure 
aluminium. In most of the excejDtional cases the other compensators were provided 
with points, and the three-point method was used, and afforded the results which 
have already been stated to accord with those where points were not used. The 5‘25 
compensator gives excellent interference bands, particularly from one of the two 
surfaces, which was marked and invariably used. The bands afforded by it were 
slightly curved, due to infinitesimal convexity, an additional advantage as it was 
always possible, by noting whether they moved outwards from or inwards towards 
the centre of curvature, to at once ascertain whether the movement of the bands 
were due to expansion or to contraction. There is a further advantage in employing 
the compensator above rather than below the crystal, namely, that the polished 
surface of aluminium reflects light almost equally with the other surface involved in 
the production of the bands, the lower surface of the large cover-glass which is laid 
on the platinum-iridium tripod screws and which bears about its centre the miniature 
silver ring whose centre is the point of reference for the micrometric measurement 
of the position of the bands. 
The air-film between the two reflecting surfaces was in nearly all cases very thin ; 
it was not found advisable to strive so much for exact compensation for the expansion 
of the screws as to produce the most brilliant bands. For the correction for non¬ 
compensation is of course in all cases accurately determined from the known expansion 
of the tripod alloy and aluminium. The screw-length corresponding to 5’25 millims. 
