322 MR, A. E. TDTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



interference bauds by monochromatic flames. The second is a train of two flint glass 

 refracting prisms, which is employed for the production and observation of the bands 

 produced, with the aid of a Geissler tube containing rarefied hydrogen and mercury, 

 by the monochromatic light corresponding to the red and greenish-blue hydrogen 

 lines, C and F of the spectrum, and to the green mercury line found so suitable by 

 PULFRICH. The interchangeability of the two is very simply attained. In the top 

 of the tubular termination of the arm, above the internal collar, there fits a very 

 much shorter tube, w, terminating above in a stout flange ; it is capable of rotating, 

 for the purpose of azimuth adjustment, in the main tube, without the possibility 

 of vertical motion, by means of an annular groove, into which project a couple of 

 screws driven through the outer tube from opposite sides. The flange, whose central 

 aperture is of slightly greater diameter than that of the porcelain tube, carries at 

 two opposite sides raised and grooved guides, in which are capable of sliding the 

 correspondingly rabbeted basal annular supports, x, of the single or double prism 

 arrangements. 



The single reflecting prism is supported in suitable bearings by central axles 

 rigidly attached to the metallic case, and provided with milled bosses at the ends for 

 convenience of rotating the prism ; this enables the latter to be adjusted for altitude, 

 and as the axles fit fairly tightly in the bearings, the setting remains unaltered after 

 adjustment. 



The pair of refracting prisms are mounted in a light framework, whose shape is 

 calculated to ensure rigidity. Each prism is cased in metal, and the case carries 

 centrally at its two sides short but thick axles, narrowing somewhat after a distance 

 equal to the thickness of the mount, and terminating in a longer screw thread. The 

 axles gear at their thickest part in corresponding bearings in the supporting 

 framework. Over the two projecting axial screws on one side of the mount are 

 passed in each case first an arm carrying at its end a silver arc as if for a vernier, 

 and which slips over the screw and narrower portion of the axle only and is 

 arrested at the thickening, and then a milled nut engaging with the thread, by which 

 the arm may be clamped firmly to the axle and hence to the prism. As the thick 

 part of the axle projects a little beyond its bearing, however, the arm is not clamped 

 to the mount. The silver arc carries a central indicating mark, and by rotation of 

 the milled head and the prism travels over a divided silver quadrant carried by the 

 mount, and thus the prism may be set to any angular reading which may at any 

 subsequent time be reproduced. The axial screws at the other side of the mount 

 receive in each case first a washer and then a milled nut similar to that on the other 

 side ; the thick part of the axle being in this case slightly shorter than the thickness 

 of the frame, the prism is clamped firmly to the mount on screwing home the nut, 

 and so the prism may be rigidly fixed to the indicated circle reading. The refracting 

 angle of each of these prisms is approximately 57, which in the case of the particular 

 flint glass employed gives a total minimum deviation due to the two of 90 for the 



