MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DII^ATOMETER. 347 



readings of the micrometer drum, the one which moves both spider-Hues simul- 

 taneously, are taken, corresponding to the setting of each of these five bands in turn, 

 precisely midway between the two spider-lines. It is convenient to call these values 

 a, b, c, d, e, letting a be the one most to the left ; the values will then be in ascending 

 order of drum divisions, each complete revolution of the drum being 100. In addition 

 to the four direct values for the width of a band obtained by taking the differences 

 between successive readings, a considerable number of other combinations can be 

 obtained for the width from the five readings, and PULFBICH has shown that the 

 whole of the values reduce eventually, converting his symbols into the author's, to 



the simple formula : 



w = i (d + e) - (a + b). 



To determine the distance of the middle band, c, from the reference point, it is only 

 necessaiy to determine the true position of c by taking the arithmetical mean of the 

 five readings, and to determine the drum reading of the reference point by taking 

 the mean of two or three independent readings corresponding to the adjustment of the 

 silver ring so that equal small arcs are visible outside each of the vertical spider-lines. 

 The difference between these mean readings for the band c and the centre of the 

 reference ring is evidently the distance S required. The fraction of a band corres- 



^s 



ponding to this temperature is, then, either S/iv or - , according to the direction 



in which the bands are found to move on raising the temperature. 



The Fletcher ring gas-burner is then ignited, the gas supply to it being regulated 

 by the graduated lever tap, so that the temperature of the bath may not rise too 

 rapidly. As has already been explained, the author prefers to actually observe the 

 passage of the bands, and to count their number with the aid of the recording 

 apparatus, rather than to rely exclusively upon the ABBE method of calculation from 

 observation with two wave-lengths. Moreover, in the case of the determination of 

 the expansion of the tripod, the bands are so very much clearer with green mercury 

 light than with either C or F hydrogen light, or sodium light, at the necessarily long 

 distance apart of the two reflecting surfaces, that the author prefers to base his 

 determinations entirely upon observations in green mercury light. The recorder is 

 placed upon the little cloth-covered accessory table to the observer's right, and as 

 each dark band (after the first, which has the determined fractional value) advances 

 to the middle of the two vertical spider-lines, the knob is pressed and the puncture 

 made in the tape. 



For a reason which will be fully discussed in considering the mode of calculation of 

 the experimental results, the author makes two series of observations, the upper 

 temperature limits of which are in the neighbourhood respectively of 70 and 120. 



The rise of temperature is so regulated, that fully an hour is occupied in attaining 

 the neighbourhood of the first higher limit, about 70, the transit of the bands 

 occurring with more or less regularity during this interval. A further valuable 



2 Y 2 



