344 MB. A. E. TUTTON ON A COMPENSATED INTERFERENCE UILATOMETER. 



venient one. The two vertical spider-lines are then to be arranged at such a 

 mutual distance as best facilitates the setting of a band to their centre, and if the 

 width mentioned is chosen, the spider-lines are then also at the most suitable 

 distance apart to enable the centre of the reference mark the inner edge of the 

 silver ring of the ordinary cover-wedge, or of the clear ring of the silvered cover- 

 wedge used in the observations of the tripod expansion to be located ; in either 

 case the desirable small segment of the circle is visible outside each spider-line when 

 the ring is set for a drum-reading of its position. The horizontal spider-line is to be 

 arranged as a tangent to the ring at the lower end of its vertical diameter. This is 

 achieved by use of the rack and pinion of the pedestal. 



The surface of the author's platinum-iridium tripod, truly plane to all ordinary 

 tests, such as reflection of goniometer signals, proves to be very slightly concave, the 

 bands in green mercury light being slightly curved, that is, arcs of large circles. 

 They are perfectly regular, however, and move parallel (concentrically) to themselves 

 when the temperature is altered, demonstrating the fortunate perfect equality 

 of expansion of the whole of the parts of the tripod, and particularly of the screws, 

 and enabling excellent determinations of the expansion to be made by observations 

 of the number of bands passing the centre of the reference circle. A width of band 

 of about GO drum divisions was found most suitable in these determinations of the 

 expansion of the tripod itself'. 



The preliminary adjustment having been carried out, the screws of the tripod are 

 rigidly fixed by tightening the little clamping screws. The bent thermometer is 

 then to be carefully attached. It is held near its upper end by a pair of loops at 

 the ends of a circular brass spring collar, fitting just under the flange at the base 

 of the prisms. This mode of support admits of sufficient play to enable the half of 

 the lid, through a hole in which the thermometer is to pass, to be slid over the bent 

 lower end and up the stem into position, and the half lid is supported by the upper 

 end of the interference chamber while the bulb of the thermometer is carefully 

 arranged in proper contact with the upper surface of the tripod table and one of the 

 screws ; the bulb may then be secured in this position by means of silk thread. 

 After ascertaining that this operation has not impaired the adjustment, or correcting 

 any slight derangement, the expansion apparatus is smoothly removed, by means 

 of the handles of the cloth-lined slab upon which it is mounted, to its marked 

 place at the further end of the slate table. The micrometer is detached, and 

 its movable lens extracted, unscrewed from its short tube, and fitted to its longer 

 tube, which is then pushed home in the tube of the micrometer until the ends 

 are flush ; the micrometer is then placed in position, being now in a condition to 

 sharply focus the bands and reference mark for the new position of the expansion 

 apparatus. The height of the telescope will require slight alteration, and a little 

 further adjustment for azimuth may be necessary ; removal of the front eyepiece 

 will, as before, enable the double image to be properly adjusted. The height 



