364 MB. A. B. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



readily be done by making the adjustment by screwing further out for the required 

 minute distance one, or, if necessary, two of the screws, rather than by slightly 

 withdrawing the opposite screw. The final measurement of the actual screw-length, 

 after the completion of the observation, affords the information required for the 

 calculation of the exact amount uncompensated. This minute amount is most 

 conveniently taken into account in the calculations in the form of a correction to 

 the change of thickness of the air-layer, or, which is the same thing, amount of 

 expansion or contraction of the crystal. If, as usual, the crystal expands with rise of 

 temperature, the correction is positive ; the effect of the slight apparent expansion of 

 the screws being to reduce the observed amount of approach of the two reflecting 

 surfaces ; if the crystal is a contracting one along this particular direction, the 

 correction requires to be subtracted, as the effect is to enhance the apparent contraction. 

 The amount of the correction is the difference between the calculated amounts of 

 expansion of the screws and the compensator ; these are obtained by multiplying the 

 measured screw-length and the thickness of the aluminium block respectively, by the 

 now-ascertained mean coefficient of expansion of the tripod alloy and aluminium, for 

 each of the two intervals of temperature, and by the number of degrees of rise of 

 temperature, in each case. 



The mode of carrying out the determinations of the number of bands which make 

 their transit during the two intervals of temperature is precisely similar to the 

 procedure which has been described for the cases of the determination of the 

 expansion of the aluminium compensator and of the glass of the covering-discs. 



In a subsequent memoir, the author hopes to present to the Royal Society the 

 results of a series of -determinations, carried out in this manner, of the thermal 

 expansion of the crystals of the sulphates and selenates of potassium, rubidium, and 

 caesium, an investigation which is now engaging his attention. Further details, 

 therefore, as to the mode of carrying out determinations with crystals, and con- 

 cerning the preparation of suitable crystal blocks, will be left for consideration in 

 that memoir. 



The dilatometer and its accessories have been constructed for the author in an 

 altogether admirable manner by Messrs. TROUGHTON "and SIMMS. 



