328 MR, A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



out of the field of view. This is achieved partly in the mount of the achromatic 

 combination itself, and partly as a final adjustment, by screwing the mount, not to 

 the draw tube itself, but to a cap or short tube, s, slipping so easily over the latter 

 that when a pair of screws are passed through both at opposite ends of a horizontal 

 diameter, the cap is free to tilt slightly about them as an axis. By means of another 

 pair of screws at the ends of a vertical diameter, passing through the cap tube only, 

 the latter may be fixed at the slight tilt required. The objective cap is further 

 provided, for use when desirable, with an outer lengthening tube not shown in the 

 figures, projecting outwards 4 centims. beyond the objective, in order to shield it 

 from any extraneous side light. 



The reflecting prism, I, is cased in metal on the hypotenuse side towards the eye- 

 piece. Between the glass surface facing the objective and the fixed diaphragm there 

 is just room to insert, through a niche cut in the outer tube, any one of the series of 

 interchangeable stops, k, suitable guides being provided to prevent actual contact 

 with the glass during the insertion or withdrawal. The stop is in its proper position 

 when it has been pushed home, as far as a handle at the top will permit. The 

 rectangular aperture is placed as near one edge as possible, and when the stop is in 

 position this edge is almost identical with the vertical diameter of the large circular 

 diaphragm aperture, and consequently with the common edge of the semicircular free 

 aperture and the prism ; also the centre of the stop lies on the horizontal diameter of 

 the circular aperture. Hence the axis of the bundle of rays incident from the rect- 

 angular signal aperture is as nearly as possible identical with the optical axis, and if 

 the rays reflected from the interference apparatus are brought to a focus in the free 

 aperture, so that the image of the signal is almost in contact with this same vertical 

 diameter, the two paths are practically identical, and the best conditions for inter- 

 ference attained. The apertures of the stops vary from 2 millims. by 1 millim. to 

 3 by 5 millims., and two series are provided, one with the longest sides of the 

 apertures horizontal, and the others with them vertical ; the former are naturally 

 preferable for use with the refracting deflection apparatus of the dilatometer. The 

 smallest size is, of course, preferable, provided the source of light is sufficiently 

 powerful. With the author's Geissler tube the illumination of the bands is many 

 times more powerful for C or F light than it is when using a sodium flame. An 

 additional stop is also provided, which is furnished with a means of varying the 

 vertical height of the aperture, and which is intended for use as a fine slit. At that 

 position in the metallic strip, which will be irfthe horizontal diameter of the field of 

 the telescope when the stop is in position, a rectangular aperture of 4 millims. width 

 is commenced, and continued upwards as far as and right through the handle. This 

 long aperture runs, of course, parallel to the edges of the strip, and very close to the 

 edge on the side which is to be nearest to the optical axis when in position. Through 

 the handle a movable strip, the same width as the aperture, but longer, so as to 

 project through the handle, passes, adequately guided by miniature guides in the 



