314 MB. A. E. TCJTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



DESCRIPTION OF THE APPARATUS. 



The Interference Tripod. 



The first essential of an interference dilatometer is the tripod support for the 

 object and for the glass lens or plate to be suspended, without touching, above the 

 latter. The upper surface of the object and the lower surface of the glass are 

 the two surfaces, by reflection of pure monochromatic light from which interference 

 curves or bands are to be produced ; and it is the movement of these bands upon 

 alteration of the temperature which affords the measurement of the change in 

 thickness of the air space which separates the two surfaces. For the passage of two 

 successive curves or bands past any given spot determines precisely that a change 

 of thickness equal to half a wave-length, of the particular light employed, has 

 occurred during the interval. 



The tripod employed by BENOIT was constructed of platinum-indium ; that of 

 ABBE and PDLFRICH was of steel. The tripod of the apparatus now described is 

 constructed of platinum-iridium, which, in addition to its obvious advantage on 

 the score of unalterability, and its exceptionally small coefficient of expansion, 

 is specially suitable for a reason which will be apparent when the method of compen- 

 sation is described. It is represented in fig. 1. 



Fig. 1. 



It is entirely constructed out of a single homogeneous casting of the alloy of 

 platinum with 10 per cent, of iridium, which was supplied by Messrs. JOHNSON and 

 MATTHEY for the purpose, in order that the thermal expansion should be uniform 

 throughout its whole extent. It consists of a solid horizontal table 7 millims. 

 thick, circular in shape, but with three short equi-distant projecting arms, through 

 which are threaded the three screws, of very fine pitch, each 35 millims. long, 

 upon the bluntly-pointed upper ends of which the glass cover-wedge is to be 

 supported. The distance between the centres of any two adjacent screws is 

 30 millims. The screws are provided, near their lower rounded ends, with milled 



