MR. A. K. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 331 



induction discharge, corresponding to a 5-centim. spark in air, leaks slightly along 

 the metal of the observing apparatus, and if no insulator is interposed, small sparks 

 are discharged from the eyepiece to the observer's eyebrows. The thick ebonite 

 disc acts as a perfect insulator, and prevents this disconcerting occurrence. 



The ordinary eyepiece, m, slides in a short tube which is directly screwed to the 

 flange, I, the amount of slide being adequate to enable the observer to sharply focus 

 the vertical edge of the reflecting prism, h, which divides the iris diaphragm into 

 two halves, a semicircle of light (clear aperture) and a dark half covered by the 

 prism. 



The micrometer optical combination, n, slides for about 3 centims. in a similar 

 but slightly longer tube attachment to the flange, it is then prevented from sliding 

 further by the thickening of the tube. This thickened part eventually passes into 

 a still thicker one, which is directly attached to the micrometer box, a ; in front 

 of the latter is attached a short tube within which slides the Ramsden double 

 eyepiece, o, which focusses the micrometer spider-lines. Three such eyepieces, of 

 graduated magnifying power, are provided, suitable for all the widths of interference 

 bands likely to be employed. As the movable lens -p is only required in two positions, 

 one corresponding to the close approximation of the expansion apparatus and the 

 telescope, and the other to their removal to the two ends of the slate table, two 

 inner sliding tubes are provided, of respectively 15 and 57 millims. length, each 

 carrying at one end an inner thread corresponding with one on the lens mount. 

 When the lens is attached to the shorter tube, and the latter is pushed in the long 

 tube of the combination until the ends of the two tubes are flush with each other, 

 the combination, when in position, focusses the bands for close quarters ; when the 

 longer tube, p ' , is employed, and similarly pushed in until flush, the lens being then 

 in the beginning of the thickest part of the long tube of the combination, the bands 

 are focussed when the expansion apparatus and the telescope are separated at 

 opposite ends of the table. If angle marks are placed on the table as indicators for 

 the position of the corners of the movable slab upon which the expansion apparatus 

 is mounted, corresponding to the sharpest focus under the two conditions, it is easy 

 to bring the slab exactly to one or other of these positions. 



The micrometer employed by the author is one in which the spider-lines are 

 moved, not the eyepiece as in the Abbe apparatus. Moreover, it is a double one, 

 carrying a divided drum on each side of the box. Each drum is divided into 

 100 parts, every 10 being figured. It may be set so that the zero corresponds 

 exactly to any desired position of the spider-line or lines by means of a milled 

 clamping boss carried at the end of the axle, which is here of square section ; the 

 aluminium drum itself, being on a round part of the axle, is free to move independently 

 of the spider-lines until the boss is pushed firmly against it and fixed there by a 

 screw driven into the axle, which does not extend so far as to be quite flush with 

 the boss ; the drum is then held firmly between the boss on the outside and 



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