MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 321 



in the rigid base ; rotation is prevented by a slot and pin. The rigid base and the 

 flange are bound together by means of a large milled nut, o, which has above an 

 inner flange projecting over the chamber flange just referred to, and whose thread 

 engages with one turned in the periphery of the base. 



The interference chamber is supported at the lower end of a non-conducting tube 

 of Berlin porcelain,^?, supported from an arm, q, carried by a stout gun-metal columnar 

 pedestal with thiee-legged base, provided with levelling screws i-esting on toe-plates. 

 The column is provided with a strongly-made vertical adjustment by rack and pinion, 

 worked by a milled ebonite disc, and provided with an arresting clamping screw, 

 manipulated by a lever to securely fix it at the required height. Sufficient rack is 

 given to enable the interference chamber to be 1'aised above the height of the air 

 bath, and conversely to be readily lowered into the latter. Particular cave has been 

 taken to render the working of the rack smooth and rigid, in order that no movement of 

 the adjusted interference apparatus shall occur during the operation. The arm is rotat- 

 able about the upper part of the column, and can be fixed when desirable by a clamping 

 screw. The outward end of the arm carries, as part of the same casting, a short thick- 

 walled tube, r, of the same diameter as the interference chamber ; at the lower end of 

 this tube the porcelain tube is supported. In order to be prepared for the possible occa- 

 sional fracture of a porcelain tube during heating, half-a-dozen such tubes were specially 

 prepared, from the author's pattern, at the Berlin porcelain works. They are glazed 

 outside and biscuit within, and have each a flange at either end ; by subsequent 

 trimming in the lathe the flanges have all been reduced exactly to the same size, so 

 that a fractured tube can be readily replaced by another. This is also facilitated by 

 the method of suspension, whieh is likewise one that renders fracture very improbable, 

 as it admits of freedom of expansion. The flange, s, passes easily up the metal 

 supporting tube carried by the arm, and after its insertion the two halves of a collar, 

 t, are passed up the metal tube under the flange until they are flush with the lower 

 end of the tube, when they are secured by screws passing through the tube and 

 collar from the outside. In order that the porcelain tube, thus easily suspended, may 

 be sufficiently rigid for the purpose in view, slight pressure is brought to bear on the 

 flange from above by a pair of circular bent springs, u, confined between the flange 

 and another internal collar, v, in the upper part of the metal tube. The lower flange 

 of the porcelain tube is similarly attached to a short metal tube, which carries inside, 

 at its lower end, an adequately long screw thread, which engages with the one at the 

 upper end of the interference chamber, by which means the latter is attached. 



The incident light rays are directed into the expansion apparatus from the illumi- 

 nating apparatus, and the reflected rays back into the latter (which, being arranged 

 for autocollimation, serves also as observing apparatus) by means of one of two inter- 

 changeable pieces of deflecting apparatus. The first is a single total reflecting prism ; 

 this is employed during the preliminary adjustment of the interference apparatus, 

 with the aid of ordinary white light, and also when it is only desired to produce the 

 VOL. cxci. A. 2 T 



