ME. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETKR. 339 



The tenths of millimetres are also indicated by the teeth of a serrated diaphragm, 

 which forms the left edge of the field of the micrometer eyepiece, and whose centre, 

 corresponding to the zero of the scale, is specially indented. The figures on the 

 drum thus represent the second place of decimals of millimetres ; and the small 

 divisions of the drum represent the third place. Hence the instrument reads directly 

 to O'OOl millim. 



ADJUSTMENT AND USE OF THE APPARATUS, AND DETERMINATION OF ITS 



CONSTANTS. 



Before the apparatus can be employed for the determination of the coefficient of 

 the absolute expansion of any substance, it is necessary to determine the expansion 

 of the platinum-iridium alloy of which the interference tripod is constructed, in order 

 that the expansion of the three screws of the tripod may be known. Having 

 ascertained this, the expansion of any substance may be determined by FIZEAU'S 

 method, provided a homogeneous specimen of the substance can be obtained in the 

 f-rm of a stout block, furnished with two parallel surfaces at least a centimetre 

 apart. For the author's purpose of being able to determine the expansion of a 

 substance with a block only half a centimetre thick, it is essential to proceed imme- 

 diately with the determination of the expansion of the specimen of aluminium from 

 which the series of compensators are cut. A block between 12 and 13 millims. thick 

 was employed for this purpose. It' the method is adopted of employing the com- 

 pensator above the crystal, these are all the data required before being able to 

 employ the apparatus directly for the determination of the absolute expansion of a 

 crystal or other small object. If the compensator is employed below the crystal, a 

 determination of the expansion of the glass of the small plates, to be used as covering 

 plates with crystals whose surfaces take a duller polish than glass, also requires to 

 be carried out. A block of the glass, 13 millims. thick, was similarly employed for this 

 purpose. The operations of making these preliminary determinations of the con- 

 stants of the interference apparatus will so fully include all the necessary manipu- 

 lative and adjusting operations in connection with the use of the apparatus, that 

 the description of the mode of conducting them, and the discussion of the mode of 

 calculating the coefficients of expansion from the experimental results obtained, will 

 at once be proceeded with. 



Determination of the Expansion of the Platinum-iridium Interference Tripod. 



This is the most difficult of all the operations in connection with the work, on 

 account of the fact that it ia necessarily performed with the two reflecting surfaces, 

 the under surface of the large glass cover-wedge laid upon the three screws and the 

 upper surface of the platinum-indium table, separated at the relatively long distance 



2x2 



