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



the thickening of the axle on the inner side. There are two movable vertical 

 spider -lines, and a fixed horizontal one; below and parallel with the latter is the 

 finely-serrated edge of a diaphragm, forming the lower boundary of the field. Every 

 fifth niche of this edge is more deeply indented, and every tenth more still. One of 

 the drums, b', moves one only of the vertical spider-lines, and thus enables it to be 

 set at any recorded distance from the other, or to measure in drum units the distance 

 of any object in the field from the fixed line. The distance between any two teeth 

 or niches of the diaphragm edge is equal to the distance traversed by the spider- 

 line for one complete revolution of the drum. The other, c, moves both vertical 

 spider-lines simultaneously, the movement from one tooth or niche to the next 

 corresponding to one complete revolution of the drum. Hence, eacli niche corre- 

 sponds to 100 drum units, and each of the deepest niches to 1000. The micrometer 

 thus affords every convenience for the measurement of the thickness of the bands 

 and the precise location of any one with respect to any point on the horizontal 

 spider-line, and will be equally suitable for any proposed method of procedure. It is 

 particularly convenient for determining the position of a band by bringing it between 

 the two parallel vertical spider-lines, which can be arranged at such a distance apart 

 as is suitable for the purpose. 



The fixed point with reference to which the position of the bands at the two 

 limiting temperatures is to be measured, and the number of bands passing which 

 during the change of temperature is to be ascertained, is of the kind adopted by 

 PULFEICH, namely, the centre of a silvered circular spot in the middle of the lower 

 surface of the glass wedge of the interference apparatus. The spot is about 1 millim. 

 in diameter, and the author has removed the central portion so as to leave a silver 

 ring, the inner circular edge of which is employed as the reference circle. Such a 

 ring is readily made by silvering the whole surface by means of a milk-sugar ammo- 

 niacal silver solution, and removing the greater portion by ordinary means, and the 

 portion about and within the ring with the aid of a rigidly supported needle and a 

 microscopist's turn-table. The spider-lines are to be placed at such a distance apart 

 as is just slightly smaller than the diameter of the inner silver circle. This enables 

 the centre of the spot to be accurately located by setting the pair of spider-lines so 

 that an equally small arc is cut off from the silver "circle on each side. The inner 

 portion is found by the author to be very advantageously left clear, as it enables a 

 band passing the centre to be actually observed at the centre. 



To illuminate the Geissler tube, the author employs a 15-centim. spark induction 

 coil, actuated to the extent of a 5-centim. spark. A 100-volt direct supply current 

 is used, filtered through five 32-candle-power lamps arranged in parallel circuit. As 

 each lamp allows 1*2 amperes to pass, the current supplied to the primary coil is 

 6 amperes, and this current affords a very satisfactory and constant illumination of 

 the tube. 



