REFRACTIVE INDICES OF THE ELEMENTS. 329 



37 centime, in length, 1 centim. in diameter, and about '15 centim. in thickness 

 of wall. Into their ends were fitted discs of ordinary plate glass, 9*2 millims. thick, 

 ground slightly conical. Alxnit f of the thickness of the plate was in contact with 

 the wall of the tube, the remaining $ projecting beyond its end. At the middle 

 point of the tul>e, and at right angles to it, a side tube of smaller diameter was 

 sealed, and, close to the junction, the side tul>e was contracted, so that it could be 

 sealed off easily after being exhausted, leaving a very short " appendix " which would 

 not project beyond the inner circumference of the electric furnace. After thoroughly 

 cleaning the tubes a pair of discs was fitted in (one in each tube), and one of the two 

 was rotated till interference bands could be seen in the field of the telescope. The 

 other pair was then similarly adjusted.* 



The tubes were then heated to about 120 C., and a thin layer of shellac was laid 

 round the junction of tube and disc. The tulxjs were next exhausted and re-heated, 

 and the shellac allowed to cool and harden under external pressure. Prepared in 

 this way, the junctions stood the effect of both temperature and pressure perfectly 

 for some time. After about a dozen experiments, the shellac would become slightly 

 porous, but this was easily remedied by a fresh coating. At the first heating, shellac 

 softens at about 120 C., but, after prolonged heating, it appears to lose its more 

 volatile constituents, and sets into a hard, almost black, mass, which adheres to glass, 

 and does not soften or char below 300 C. 



A weighed quantity of the substance whose refractive index was to be measured 

 was then introduced into one of the tubes by the side tube, with the precautions 

 demanded by its chemical nature which are given below. The tube was evacuated 

 with a Fleuss pump, and the side tube sealed off as close as possible to the 

 main tube. 



The other tube was not sealed off, but was left in connection with a pump and 

 manometer throughout the experiment, so that a leak, if any occurred, could be 

 detected at once. 



The two tubes were laid side by side, enveloped in a jacket of iron plate, which 

 served to equalise the temperature, and to hoM them together. They were then placed 

 on two crescents of asbestos which fitted the inside of the furnace, and maintained 

 them symmetrically in the centre of the cylinder, equidistant from its sides. The 

 two halves, in which the electric furnace was made, were now drawn together over the 

 tul>es, and the junction carefully packed with asbestos, a thermometer, for rough 

 observations of temperature, being inserted with its bulb in contact with the glass 

 of the tubes at their middle point, which was usually the coolest portion of the 

 system. Finally, the whole was adjusted between the mirrors of the refractometer 

 till interference l>;m<ls could be seen in the field of the telescope. A pointer was 

 hung on the mirror furthest from the eye to mark the zero of observation. 



* This adjustment is necessary owing to the fact that it is very difficult to grind a stopper so truly that 

 the axis of the cone which it forms is accurately at right angles to the plane of the plate. 

 VOL. CCIV. A. 2 U 



