348 MR. A. E. TTJTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



control of the number of bands passing the spider-lines is afforded if the observer 

 notes the passage of each quarter of a band in his note-book. The author calls the 

 quarters respectively 1, 2, 3, 4, and as the four successively make their transit, puts 

 down their number on the same line ; after 4 is so put down, the puncture is 

 immediately made on the recorder, and a " p " added after the 4 in the note-book, to 

 signify that the puncture has been made. If, also, the number of this particular 

 band is added, an additional check is obtained. It may be mentioned that, in the 

 case of the tripod determination, and all other cases where the bands are not so 

 numerous as to pass with great rapidity, it is not necessary to keep the induction 

 coil in action during the whole time ; it is sufficient to run it intermittently in such 

 a manner as to observe the passage of the four quarters of each band. This prevents 

 overheating of the Geissler tube, and considerably prolongs its efficiency. The 

 hydrogen spectrum rapidly deteriorates if the coil is worked continuously. A further 

 period of two to three hours is then allowed for the attainment of equilibrium at this 

 temperature, during the last hour and a half, at least, of which the temperature is 

 maintained constant with the aid of the thermostat and the graduated gas-tap. It 

 is found that if constancy to within 0'2 is attained in the inner bath, as indicated 

 by the outer two thermometers, a degree of constancy fairly easy to maintain, 

 the actual temperature of the interference apparatus, as indicated by the inner bent 

 thermometer whose bulb is in direct contact with the platinutn-iridium tripod, 

 remains absolutely constant. For the last ten degrees or so of the rise of tempera- 

 ture, the bands move precisely with the mercury column of this thermometer, and 

 their motion ceases exactly with the attainment of constancy by it, a circumstance 

 which is eminently satisfactory. When the last band to pass the vertical spider- 

 lines for this temperature interval has been recorded, and no further movement of 

 the bands is observable, the tape is drawn forward some distance, in order to inter- 

 pose an interval between the last puncture for this temperature interval and the first 

 of the next, and a series of measurements similar to those at the starting temperature 

 are made, in order to determine the fraction of a band which has passed since the 

 last transit. 



When these measurements are completed, the gas supply is again increased so as 

 to gradually raise the temperature to the highest limit, about 120. The transit of 

 bands is again observed and recorded, the same interval as before is allowed for the 

 attainment of equilibrium and constancy, and finally, the determination of the last 

 fraction of a band is made in the same manner as the two former determinations. 

 The gas supply can then be turned off, and the apparatus allowed to cool. The tape 

 is again drawn forward, and as the determination is finished, cut off. It is con- 

 venient to leave ample tape on each side of the punctures for a brief description of 

 the determination to which it refers ; also to add at each end of the row of punctures 

 corresponding to each of the two temperature intervals the values of the fractions to 

 be added. In the case of the first puncture of the second interval, this only 



