334 MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



do more than slightly reduce the source of error, besides introducing an error due to 

 the impossibility of stirring the oil, on account of the disturbance which a stirrer 

 would cause to such highly delicate observations as are here in question. 



i < * 



Recording Apparatus. 



ABBE has shown that it is not necessary to actually follow and count the number 

 of bands which pass the silver spot during the interval between the two limiting 

 temperatures, it being possible to deduce the number by calculation from the data 

 afforded by observations of the initial and final positions of the bands adjacent to the 

 silver spot for light of two different wave-lengths. It is necessary, however, that 

 these initial and final positions should be determined with the utmost precision. In 

 addition to employing this method, which has been elaborately worked out by 

 PULFRICH (loc. cit.}, the author prefers to remove the slightest possibility of doubt as 

 to the number of bands which pass the reference spot by actually following and 

 counting them, as a mistake of a single band is highly important when the observa- 

 tions relate to the very slight differences of expansion between the members of a series 

 of isomorphous salts. Moreover, it is far more satisfactory to observe the transit of the 

 bands throughout, in order to be assured that it has been unbrokenly uniform, and 

 that there has consequently been no derangement of the adjustment, or cracking, or 

 irregular expansion of the crystal. In order that no error of counting may occur, a 

 method has been adopted by means of which a permanent record of the passage of 

 each band is obtained. A small recording apparatus has been constructed, of such a 

 nature that, by pressing down a key with the finger the moment each band passes 

 the silver spot, a puncture is made in a paper tape, and on releasing the key the 

 tape is moved on a short space ready for the next puncture. When the bands 

 become stationary again, on the attainment of the higher limit of temperature, the 

 tape is cut off behind the last puncture, and thus a permanent record of the number 

 of bands which have made their transit is obtained, which can be counted at leisure, 

 and verified as frequently as may be desired. 



The recorder is shown to the right on the accessory table in fig. 2. A drum, 

 carrying a roll of Morse tape, is suspended on an axle inside a rectangular box, 

 the front side of which is hinged below in order that the box may be thrown 

 open, for the purpose of replenishing the drum. For this latter purpose the axle 

 is fixed, and the drum provided with a corresponding central bore ; the drum can 

 consequently be drawn forwards right off the axle, and replenished, and is again 

 exactly in position when pushed on the axle as far as it will go, a stout boss 

 attached to a disc screwed to the back of the box arresting its further progress. In 

 order to prevent the drum coming forwards more or less off the axle during working, 

 which would pull the tape out of position, a spring is fixed to the inside of the front 

 lid, and when the latter is closed and is fastened by the two hooks and pins provided 



