MR. A. E. TCJTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 317 



happens to reflect light about equally with the lower surface of the glass cover- 

 wedge laid on the screws, and thus affords most excellent bands. The smallest 

 compensators, of 6 millims. diameter, still exhibit a field of about eight bands of 

 normal width, ample for the purpose of the observations ; the points of these 

 compensators are only 5 millims. apart, so that the method is applicable down to 

 crystals of only slightly greater extent of surface than is adequate for these points 

 to rest upon. May, 1898.] 



The Cover- Weaye. 



The thick glass disc of 40 millims. diameter, which is placed over the screws of 

 the tripod, and whose lower plane surface is to form the upper of the two surfaces 

 relevant to the production of interference, is not lenticular, as employed by FIZKAU 

 and by BENOIT, but is possessed also of an upper plane surface, as used by ABBE and 

 by PULVRICH, in order that parallel rectilinear bands and not curved ones may 

 be produced. The surfaces of this plate are not precisely parallel, but are inclined 

 at an angle of thirty-five minutes, forming in reality a wedge of extremely small 

 angle. By this device tho reflection of the illuminating light from the upper surface, 

 which would otherwise illuminate the interspaces between the bands, is deflected out 

 of the field of vision. 



General Arrangement of the Dilatometer. 



The interference apparatus which has now been described is supported during 

 the observations in a manner similar in principle to that employed by AISHK, but 

 differing in the constructive details; and the heating apparatus is an air bath of 

 special construction instead of an oil bath. Moreover, the interference tripod rests 

 on non-conducting material in direct contact with the heated air, whose temperature 

 is measured by the bath thermometers, and its actual temperature is determined by 

 a third thermometer, so bent that the bulb is in direct contact with it. Further, 

 this portion of the dilatometer, which may be termed the expansion apparatus, is, 

 unlike the ABBE arrangement, separated from the illuminating and observing 

 apparatus, which latter is removed to a considerable distance in order to be well out 

 of the range of the heated atmosphere in the neighbourhood of the air bath. 



A general view of the dilatometer and its accessories is given in fig. 2. The two 

 parts, the expansion apparatus on the left, and the illuminating and observing 

 apparatus on the right, are mounted at the two ends of a rigid slate table six feet 

 long. 



The Expansion Apparatus. 



The details of the expansion apparatus will be more readily understood with the 

 aid of the section, fig. 3. The tripod, a, stands upon a thick circular glass plate, I, of 

 50 millims. diameter, which forms the floor of the interference chamber. This 



