208 Mr. A. E. Tutton. 



IL may therefore be hoped that in the near future an understand- 

 ing will be arrived at on the importance of the development to be 

 given to the observatory of the Azores, and I ask the Royal Society,, 

 whose influence is so great in the domain of science, to support, by 

 its concurrence, the accession of England to the ideas which I uphold 

 for the common interest. 



" A Compensated Interference Dilatometer." By A. E. TUTTON, 

 Assoc. R.C.S. Communicated by Captain ABNEY, C.B., 

 F.R.S. Received March 8, Read April 28, 1898. 



(Abstract.) 



The author describes a form of Fizeau interference dilatometer 

 which he considers combines the best features of the apparatus 

 described by Benoit, and belonging to the Bureau International des 

 Poids et Mesnres, in Paris,* and that described by Pulfrich'f con- 

 structed according to the modifications introduced into the method 

 by Abbe. Moreover, besides other improvements, a new principle, 

 that of compensation for the expansion of the screws of the Fizeau 

 tripod which supports the object, is introduced, which enhances the 

 sensitiveness of the method so highly as to render it applicable 

 to the determination of the expansion of crystals in general, includ- 

 ing those of chemical preparations. Hitherto the application of the 

 Fizeau method has been confined to such crystals as could be obtained 

 large enough to furnish a homogeneous block at least a centimetre 

 thick. A block only 5 mm. thick is ample for use with the author'* 

 compensated dilatometer. The principle of the compensation de- 

 pends upon the fact that aluminium expands 2'6 times as much as 

 platinum-iridium for the same increment of temperature. The author 

 therefore employs, like Fizeau and Benoit, a tripod of platinum- 

 iridium, and places upon its transverse table, through which pass 

 the three screws, a disc of aluminium whose thickness is l/2'6ths of 

 the length of the screws. The space between the lower surface of 

 the glass plate which is laid upon the upper ends of the screws to 

 assist in producing the interference, and the upper surface of the 

 aluminium, then remains constant for all temperatures under obser- 

 vation, and if a crystal is laid upon the aluminium compensator the 

 whole amount of its expansion by rise of temperature is available 

 for measurement by the interference method. Hence the method is 

 no longer a merely relative one, affording the difference of expansion 

 between the tripod and the substance investigated, but affords* 

 directly absolute measurements of the expansion. 



* ' Trav. et Memoires,' 1881, p. 1. 



f ' Zeita. fur Instrumen.,' 1893, p. 365. 





