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



leaving the interference tube immersed in the bath, so as also to share in the 

 desiccation. It is a great advantage if the adjustment can be carried out the evening 

 before a determination, as not only is there then ample time for the temperature of 

 the room and apparatus to attain equilibrium, but also vitriol can then be left in the 

 bath during the night, and removed just before the observations commence in the 

 morning. For if the bath and interference apparatus are long exposed to moist air 

 during adjusting operations, and the tripod subjected to much manipulation with the 

 fingers, condensation again occurs to a greater or less extent which it is desirable to 

 remove. The fixing of the mica screen should be left till this is carried out. The 

 momentary raising of the interference apparatus from the bath in order to remove 

 the vitriol dish serves, moreover, to enable the observer to note the reading of the 

 inner bent thermometer, whose bulb is in contact with the tripod. This temperature 

 is, of course, taken as the starting temperature in preference to that indicated by the 

 bath thermometers, in case any slight difference is apparent. 



Finally, before commencing actual observations, the bands are closely examined, 

 and any slight alteration of the height of the reference ring above the horizontal 

 spider-line corrected ; also the eyepiece should be momentarily removed to satisfy 

 oneself that the double- image is still correctly placed. The observation of the 

 position of the bands for the starting temperature can then be proceeded with. 



The method adopted for determining the position of the bands, with reference to 

 the centre of the silver ring, is that recommended by PULFIUCH. Two things require 

 to be ascertained. First, the distance between the centres of two adjacent dark 

 bands, which may be termed the width of the bands ; and second, the distance 

 between the centre of the nearest dark band and the centre of the reference ring. 

 The quotient of the second by the first is evidently the fraction of a band by which 

 the centre of reference is distant from the nearest band. When the temperature is 

 raised, and the bands move past the vertical spider-lines, the first dark band which 

 comes between the pair of spider-lines is to be counted, not as one band, but as the 

 fraction of a band thus determined, in case the movement of the nearest band occurs 

 in the direction of the centre ; and in the contrary case as the complementary 

 fraction, that is, unity minus the determined fraction. The whole observation is 

 completed by counting or otherwise determining the number of succeeding dark 

 bands which pass during the interval until the temperature again becomes constant 

 at the higher limit, and by then determining in a similar manner as for the lower 

 limit the fraction of a band to be added, beyond the last which has passed the centre. 

 Hence, the essentials of the determination are the whole number of bands which pass, 

 and the initial and final fractions of a band to be added to this whole number ; and 

 the determination of each of these fractions involves the two operations previously 

 referred to. 



To determine the width of the bands the author measures the five whose middle 

 one is immediately to the left of the centre of the little reference ring. Duplicate 



