MR. A. E. TCTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 325 



aperture, and a totally-reflecting prism, h, covering one-half of the aperture and so 

 arranged that the rays from the illuminating lens, d, are reflected through this half 

 of the aperture towards the objective, b, which latter they are capable of filling with 

 light. The surface of the prism nearest the objective is intended to be masked by 

 one or other of a series of interchangeable stops, k, having larger or smaller 

 rectangular apertures. A short distance after the prism comes a stout milled 

 flange, I, to the thread within the aperture of which either of two interchangeable 

 observing optical arrangements is capable of being attached. One, m, is an ordinary 

 eyepiece, for use in adjusting the interference apparatus by observation of the images 

 of the rectangular signal aperture, k, in front of the reflecting prism, reflected from 

 the various reflecting surfaces of that apparatus. The other, n, is an optical com- 

 bination of a micrometer two-lens eyepiece, o, with a third movable lens, p. The 

 system is such that when the lens p is at that end of its path nearest the prism, the 

 reflecting surfaces of the interference apparatus and the interference bands which they 

 exhibit are clearly focussed to the observer looking through the eyepiece o, when the 

 expansion apparatus is approached very near to the observing apparatus ; and when p 

 is at the end of its range nearest to the eyepiece o, the same result is achieved for 

 the separation of the two parts of the apparatus by about five feet. There is no 

 necessity for a separate adjusting apparatus, as used by PULFRIOH, for when the first 

 of the two conditions just referred to obtains, the two parts of the apparatus are so 

 close together that it is quite easy to manipulate the screws of the interference tripod 

 while actually observing. The relatively large windows of the interference chamber, 

 as distinguished from PULFRICH'S closed chamber, greatly facilitate this. 



For convenience in moving the expansion apparatus between the two positions, the 

 toe-plates of its pedestal do not, like those of the telescope, rest directly upon the 

 long slate table upon which -the instrument is mounted, but on a smaller rigid slab of 

 hard mahogany, lined underneath with thick baize, and furnished with a couple of 

 stout brass handles. 



In addition to the vertical rack and pinion motion provided in the pedestal of the 

 telescope, the latter can be adjusted for altitude by a tilting arrangement. It is 

 mounted about trunnions and the axis of rotation is made coincident with that of the 

 illuminating side tube, which latter forms a prolongation of the hollow axle on one 

 side. This is convenient when employing a source of light which does not move 

 with the apparatus as does the Geissler tube, the illuminating centre remaining in 

 the prolongation of the axis of the side tube whatever the tilt of the telescope. The 

 latter may be laid in the bearings with the illuminating tube on either side according 

 to convenience ; it is only necessary to remove the two screws which in each case 

 secure the caps of the bearings in order to effect a change of side. The telescope is 

 suspended sufficiently high above the transverse part of the bearing casting to admit 

 of considerable inclination from the horizontal position, and the depression or eleva- 

 tion is effected by a milled-headed screw which passes through a rigid arm radiating 



