316 MB. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



should not have to be rejected as unsuitable because either one or both of the 

 parallel horizontal faces, perpendicular to which the expansion is to be measured, is, 

 relatively to the other dimensions of the crystal, small. For these reasons the 

 author usually lays upon the crystal a circular plate of glass, sufficiently thick to be 

 adequately rigid, the lower surface of which is ground dull and the upper polished 

 and worked perfectly plane. The glass employed is black astronomical glass, in 

 order that no disturbance may arise from an illuminated lower surface, as would be 

 more or less the case with colourless glass. The expansion of this glass, which has 

 been accurately determined, is very nearly the same as that of platinum -iridium, so 

 that the very slightly coirected measure of the thickness of the plate has simply to 

 be added to the length of the screws calculated for compensation by the aluminium, 

 and the screws sot to this total length. Three series of such discs, of respectively 

 25, 15, and 10 millims. diameter, have been provided for use with crystals of greater 

 or iess lateral extension ; for it is unwise to employ the 25-millim. disc with 

 crystals of small diameter, a columnar type placed on end for instance, and the 

 smaller ones still provide an adequate field of bands. One of medium size is shown 

 resting on a crystal in fig. 1. Each series consists of three plates, varying from 

 2 to 4 millims. in thickness, so that there is ample choice afforded. Such a choice is 

 preferably made as will leave, when the crystal and compensator are in position, an 

 air space, between the upper surface of the glass plate and the lower surface of the 

 large disc resting on the screws, of less than half a millimetre, in order that brilliant 

 interference bands may be obtained. The size mostly employed by the author with 

 crystals of artificial salts is 10 millims. diameter and 2 millims. in thickness. 



[The second method of using the compensator, namely, above the crystal, is the 

 one which the author prefers to employ whenever possible. Three series of 

 aluminium discs are provided for use with this method, of respectively 15, 10, and 

 6 millims. diameter. Each disc is polished as truly plane as possible on one surface, 

 intended to be the upper surface when in position on the crystal, and carries on 

 the other side three equi-distant points, the ends of miniature screws of the same 

 aluminium firmly screwed into the disc. Each series consists of two discs, the 

 thicknesses of which, including the points, are respectively 4 and 5 millims. One 

 of these compensators is shown resting on the ground in fig. 1. By this method 

 the lower surface of the crystal rests directly on the three points of the 

 platinum-iridium table, the particular three varying with the size of the crystal but 

 being usually the innermost three, and the aluminium compensator rests by its 

 three points upon the upper surface of the crystal. Hence any movement due to 

 very slight convexity of the surfaces, or possible rolling upon a speck of dust 

 included between the surfaces, is entirely avoided by this arrangement of three-point 

 contact only. Moreover, the calculations are simplified and sources of error reduced 

 by avoiding the use of a glass crystal-covering disc. In this connection it is par- 

 ticularly fortunate that aluminium does not take a very high polish, but one which 



