MB. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 327 



than the dimensions of the arm at this point, affording means of adjustment by four 

 small adjusting screws driven through the top of the slider, whose points are pulled 

 tightly down upon the arm by two larger milled-headed binding screws passing 

 through the under side of the slider, one on each side of the scale. By means of 

 these screws the scale is readily adjusted to its proper position with respect to the 

 indicating flange edge. The scale is 60 millims. long, this distance corresponding to 

 the traverse by the telescope of rather more than the whole length of the spectrum. 

 Hence a delicate means is afforded of setting the telescope to the proper tilt for the 

 use of either C or F hydrogen 1'ght, or that corresponding to the green mercury line, 

 without actually observing the signal image for that colour at every observation. It 

 is only necessary to ascertain once for all, with occasional verifications, the scale 

 readings corresponding to these colours. 



The bearing casting of the telescope carries below a vertical conical axis, which 

 rotates within a corresponding hollow cone or vertical bearing, thus admitting of 

 rotation of the telescope in the horizontal plane ; the greater portion of this vertical 

 conical bearing passes down into the wide internal boring of the pedestal, and it is 

 rigidly attached to the latter by a screw thread turned on the thickened upper part 

 of its cylindrical exterior and engaging with one in the upper part of the boring of 

 the pedestal. The telescope is prevented from rising in the vertical bearing by a 

 screw driven into the lower end of the solid cone, and a washer broad enough to cover 

 also the end of the bearing. This vertical bearing carries at the top as part of the 

 same casting, immediately above the thread by which it is attached to the pedestal, 

 a broad collar, passing on one side into an arm, intended for use in connection with a 

 fine adjustment fitting placed immediately above it. This latter takes the form of 

 another collar passing round the thickened upper end of the conical axis and con- 

 tinued on two opposite sides into arms. That on one side is short, and through it 

 passes a milled-headed clamping screw to attach the fitting rigidly to the cone, and 

 hence to the telescope ; the opposite arm is of the same length as the fixed one 

 carried by the vertical bearing, and its end carries below two elbow pieces, forming 

 two vertical claws, between which the narrower end of the fixed arm passes, with the 

 necessary amount of free play within which the fine adjustment can be effected. The 

 end of the fixed arm is pressed between the fine adjustment screw passing through 

 one claw, and a spring piston carried by the other. Hence, in order to effect fine 

 adjustment for azimuth, it is only needful to tighten the clamping screw, and then to 

 manipulate the fine adjusting screw. 



The telescope is provided with a rack and pinion adjustment for the objective, 

 which is of 4 centims. aperture, and is placed at the end of an inner tube, q, which 

 carries the rack, r. This enables the objective to be brought exactly to the focus of 

 the signal aperture of the reflecting prism. There are also provided the means of 

 slightly tilting the objective, in order to throw the troublesome reflection from the 

 first concave lens surface just behind the diaphragm of the micrometer, and hence 



