330 ME. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



attached tube. At the outer end of the movable tube is carried a three-disc center- 

 ing apparatus, y ; the second and third discs are complementarity pivoted about 

 points near their periphery, the second about the first, and the third about the second ; 

 they are maintained rigidly in position by similar pins on the other side of the centre, 

 working in slots, and their movement is effected by a pair of screws and spring 

 pistons, one carried by the first and another by the third disc, working against pro- 

 jections carried on each side of the middle disc and which are movable in larger slots, 

 cut likewise in the first and third discs. The three discs are pierced by central aper- 

 tures sufficiently large to admit the rays from the capillary to the lens. Attached to 

 the third disc is a vertical tubular holder, 2, for the Geissler tube, one limb of which 

 slides easily in it, a slit being provided in its outer side for the passage of the capillary 

 as far as the centre. Owing to the presence on this limb of some of the author's 

 Geissler tubes of a little sealed side tube, which had been used for the purpose of the 

 exhaustion, the sliding into position of the Geissler tube is brought about from 

 below. When the capillary is raised as far as the slit permits, it is approximately 

 central ; a loop support is then attached below it to maintain it in position, by means 

 of a pair of milled-heacled screws, which secure the loop to the stout bracket which 

 attaches the tube holder to the third centering disc. The inner side of the tubular 

 holder has a central aperture corresponding to those of the three discs. The bracket 

 is attached in such a manner to the third disc as to provide a means of adjustment 

 for altitude, the claws, one on either side of the holder, not being secured by the 

 pair of screws directly in contact with the disc, but being able to swivel more or less 

 about a horizontal axis formed by a pair of short pins, one carried by each claw 

 between the two screw holes and on the side which would otherwise be in contact 

 with the disc. The slit in which the capillary slides being adequately large, adjust- 

 ment for azimuth is readily attained. Hence every required adjustment of the 

 capillary is provided for. In order that the Geissler tube may be firmly held, and 

 also that the glass may not be in direct contact with the metal tube, a pair of 

 caoutchouc rings of suitable thickness are introduced as packing, one near the top 

 and one near the lower end of the holder. As the Geissler tube fitting requires to 

 be frequently removed, and it cannot be laid down without the risk of mercury 

 getting into the capillary, an inclined cloth-covered supporting pillar, on a suitable 

 stand, is provided for its reception when not in use. The pillar is of slightly less 

 diameter than the inner tube of the fitting, so as to pass inside it until arrested by 

 the centering discs. The Geissler tube can thus be always left in position in the 

 fitting. It is shown on the side table in fig. 2. 



The milled flange, I (fig. 4), which completes the main optical tube, and to the 

 aperture of which the observing eyepiece or micrometer combination is attached, has 

 been constructed in duplicate, one of metal, for use whenever the Geissler tube is not 

 necessary, and another of ebonite, for use in connection with the Geissler tube. 

 For it is found that when the latter is employed, the relatively powerful electrical 



