ME. A. E. TUTTON ON A COMPENSATED INTERFERENCE D1LATOMETER. 337 



height, for the support of the object whose upper horizontal surface is to act as the 

 lower reflecting surface. The base has a much larger central aperture than the 

 ordinary one (k of fig. 3) in order to permit the passage of a stout, hollow cylinder, 

 rigidly carried below by the table ; otherwise it is similar to the ordinary one, 

 having three screws passing through it from beneath, pressing upwards against the 

 table for the purpose of adjusting the latter, which is pulled firmly down upon them 

 by a stout spiral spring, confined between the under side of the base, and a collar 

 carried by the hollow cylinder. Within this hollow cylinder slides another, also 

 very stout walled, and closed at the upper end, which is truly plane perpendicular to 

 the axis and forms the vertically movable platform. The fit of the two cylinders is 

 a very close one, and the movement is effected by a pinion, whose manipulating 

 milled head gears in a rack sunk in the movable internal cylinder. It is imperative 

 that there should be absolutely no alteration of the parallelism of the vertical axis 

 during movement, as the minutest amount of such alteration is sufficient to entirely 

 alter the position and width of the bands ; hence the closeness of the fit and the 

 relatively considerable length of the cylinders. In order that the amount of 

 separation of the two surfaces may be accurately determined, the milled head of the 

 pinion carries at its inner side a divided drum, reading directly to quarter millimetres 

 with the aid of an indicator carried by the outer cylinder. The three tripod screws 

 are adjustable for height by manipulation of the milled flanges carried some little 

 distance below the pointed upper ends, which enable the screws to be more or 

 less screwed into the fixed nuts carried on the surface of the table. The base carries 

 a screw thread on its periphery, similar to that on the ordinary one, for attachment 

 by means of the tapped flange to the lower end of the interference chamber. 



Thickness Measurer. 



The measurements of the thickness of the objects investigated, and of the various 

 accessory aluminium blocks and glass discs employed in the work, are carried out by 

 the author with the aid of an admirably accurate thickness measurer of the same type 

 as that furnished by ZEISS, for use in connection with the Abbe dilatometer, and 

 described by PULFRICH in the ' Zeitschrift fur Instrumentenkunde,' 1892, p. 307. 

 An improvement is introduced, however, into the method of suspending the 

 counterpoised vertical silver scale, which does not form the continuation of the 

 agate-pointed contact rod, but is suspended in front of the latter, in such a manner 

 as to allow of a delicate means of adjusting the zero of the scale exactly to the centre 

 of the pair of parallel horizontal spider-lines of the micrometer eyepiece of the 

 microscope, when the agate point is resting upon the glass disc fixed in the base. 



The instrument is shown in fig. 6, from which the nature of the improved method of 

 suspension will be apparent. Eound the rod, at the suitable height, a collar is fixed 

 by a clamping screw ; the collar is continued forwards and sideways into a bracket, 



VOL. cxci. A. 2 x 



