MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 345 



adjustment is best achieved while the eyepiece is in position and the bands and 

 reference circle in view, the rack and pinion of the pedestal being manipulated 

 so as to again bring the lower margin of the little circle into apparent contact with 

 the horizontal spider-line. 



The heating bath may now be placed in position upon its annular support, which 

 latter is always left in situ. To enable this to be done, the interference tube is 

 gently raised, by the rigidly moving rack and pinion, to the necessary height, indi- 

 cated by a mark on the sliding column. A large circular sheet of asbestos millboard, 

 pierced by the necessary apertures, is intended to be laid on the top, and a large 

 semi-cylindrical screen of the same material is placed between the bath and the pedestal 

 for the full height of the latter. It is most convenient that the circular sheet should 

 be arranged while fixing the bent thermometer, above the two halves of the lid, which 

 are readily supported on the upper end of the interference chamber. Several circular 

 sheets of asbestos are already laid beneath the Fletcher ring gas-burner, alternated 

 with porcelain tiles. The interference tube is then lowered into the bath, another 

 mark on the sliding column indicating the proper position, which, as adjustment had 

 designedly been made with the column at this same height as indicated by the mark, 

 will be the same as before. The two halves of the lid and the asbestos circle are left 

 in position by the sinking of the interference tube, and the wide aperture necessarily 

 left in the asbestos for the passage of the interference chamber is covered with two 

 additional overlapping semicircles of asbestos. The thermostat and the bath ther- 

 mometers may next be placed in position and the gas connections made. In order to 

 protect the dispersing apparatus as much as possible during the heating a further 

 circular screen of mica, arranged in two halves, attached by studs at each end and 

 perforated by suitable apertures at the overlapping inner edges, is fixed about the 

 height of the top of the porcelain tube. It is conveniently supported by the top of 

 the semi-cylindrical screen and a couple of asbestos supports laid on each side of the 

 front part of the asbestos circle, leaving ample space between for a clear view of the 

 thermometers. By this means the temperature at the dispersion apparatus is only 

 appreciably higher than that of the room when the thermometers are indicating the 

 highest limit of 120. 



It should be here stated that the author was much troubled at first by the 

 absorption or condensation of moisture in or on the copper walls of the inner bath. 

 It is quite unnoticeable at the ordinary temperature, but when the temperature is 

 raised and the transit of bands commences, the latter become suddenly obscured and 

 the observation is lost, owing to the condensation of a film of distilled moisture on 

 the glass wedges in the upper part of the interference chamber. This awkward 

 circumstance necessitates the abandonment of the work for the day, as the apparatus 

 requires to stand at least twelve hours to re-take the atmospheric temperature. It 

 can be completely avoided, however, by placing on the floor of the bath, after each 

 determination, so as to desiccate it before the next, a shallow dish of vitriol, and 



VOL. CXCI. A. 2 Y 





