MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 315 



heads for convenience of rotation ; and each of the arms is slit from one side, the 

 slit passing through the centre of the screw hole and slightly beyond the latter, and 

 provided with a small clamping screw, which draws the two sides of the slit together, 

 binding the main vertical screw and thus enabling it to be firmly fixed in any 

 position. Both sides of the table are polished, and on one side three concentric 

 series of raised points are provided, three equi-distant ones in each series, and each 

 series increasing in height as the centre is receded from. These serve as rests foi 

 objects of different sizes. 



The Compensation. 



It is an interesting fact that pure aluminium expands 2'G times as much as 

 platinum-iridium for the same increment of temperature, and it is this fact which the 

 author has utilised for the purpose of effecting compensation for the expansion of the 

 platinum-iridium screws. A circular disc of aluminium, whose thickness is about ten 

 twenty-sixths of the length of the parts of the platinum-iridium screws projecting 

 through and above the table, is laid upon the latter. Provided adequate care has 

 been taken to adjust the screws to the length calculated for complete compensation, 

 from a knowledge of the two coefficients of expansion, it will be evident that the 

 expansion of the apparatus is entirely eliminated. The length of screw provided 

 affords, after compensation, an available space of some millims. above the aluminium 

 block in which to place the crystal or other substance to be investigated, and as this 

 space remains unaltered by change of temperature, any alteration of the thickness of 

 the air layer between the upper surface of the object and the lower surface of the 

 glass disc laid upon the screws must be entirely due to the expansion of the crystal. 

 Moreover, the relative positions of the aluminium block and the crystal may be inter- 

 changed, it being immaterial which is uppermost. For the case in which the crystal 

 rests on the block, a series of circular aluminium blocks of 25 millims. diameter 

 and of respectively 12, 10, 8, G and 4 millims. thickness are provided, suitable for 

 use with all the various sizes of crystals which are likely to be met with. The 

 thinnest one is shown in position in fig. 1, resting upon the raised points. They 

 were constructed out of a homogeneous casting of the purest aluminium obtainable. 

 It is only necessary to select the block which is most suitable for use in connection 

 with the particular size of crystal to be employed, and to set the screws to the proper 

 corresponding length, which has been calculated, verified experimentally, and 

 recorded in a table once for all. 



The polish which many crystalline solids take upon their ground plane surfaces is 

 rarely of a-character equal to that of glass, and it is particularly desirable that the 

 two reflecting surfaces relevant to the interference should reflect as equally as 

 possible, in order that the interspaces between the bands shall be as dark as possible. 

 Moreover, it is likewise desirable that the lower reflecting surface should be fairly 

 extended, in order to afford an ample field of interference bands ; and that a crystal 



2 s 2 



