MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DTLATOMKTRR. 333 



The Thermometers. 



The two thermometers, whose lower halves were immersed in the inner bath on 

 each side of the interference chamber, were the two excellent instruments supplied 

 by FUESS, with the most recent form of heating apparatus in connection with his 

 largest gonio-spectrometer No. IA. They had previously been tested several times, 

 and twice during the course of this expansion work their zero and 1 00 points were 

 again determined with great care. The zero of both remained exact throughout, and 

 the correction for boiling-point never exceeded 0'18. Experience has shown, how- 

 ever, that the temperature of the interior of the interference chamber never quite 

 attains the temperature of the inner bath at the higher limits. The difference for 

 the neighbourhood of 70 is about 2, and for the higher limit of 120 it is rarely less 

 than 4. These bath thermometers were merely used, therefore, for the purpose of 

 attaining and maintaining a constant temperature in the inner bath. For it is found 

 that if the temperature of the inner bath can be maintained constant to 0'2, which 

 can readily be attained by the combined use of the graduated gas-tap and thermostat, 

 the actual temperature of the tripod remains constant to well within 0'l, and as it is 

 capable of exact measurement, and the bands move precisely with the temperature of 

 the tripod, all error of temperature disappears. 



The measurement of the actual temperature of the tripod is attained by a third 

 thermometer, specially constructed for the author by Messrs. NEGRETTI and ZAMBRA. 

 This thermometer is bent at right angles just above the bulb, and it is so suspended 

 alongside the interference tube that the small elbow carrying the bulb passes right 

 into the interior of the interference chamber itself, without contact with any part of 

 the walls of the chamber, and the cylindrical bulb rests in actual contact with the 

 upper surface of the platinum-iridium table and one of the screws. Special care has 

 been taken to determine its fixed points from time to time ; its capillary was a 

 specially selected one, and its zero and boiling points have required only the minutest 

 corrections. It was so constructed that the 70 mark came just outside the bath, so 

 that no correction for exposed stem is required for the lower limit. For the higher 

 limit of 120 this correction is necessary, and to enable it to be made another much 

 smaller thermometer was attached to the stem, with the bulb opposite to the 90 

 mark. That the indications of this thermometer actually express the temperature of 

 the tripod and whatever it supports, is proved by the fact that the bands follow it 

 exactly, their motion being arrested simultaneously with that of the mercury column. 

 The importance of actual contact of the bulb of the thermometer with the interference 

 tripod has been very exhaustively proved. Mere hanging of a straight thermometer 

 alongside the chamber, as in the apparatus employed by PULFRICH, affords no 

 guarantee, especially with a closed interference chamber, that the tripod and its 

 contents actually attain the temperature indicated thereby. In the author's expe- 

 rience, even with an open chamber, it never does. The use of oil in the bath cannot 



