428 A. L. Day and J. K. Clement — Gas Thermometer. 



upright brass tubes firmly secured in position upon massive brass 

 carriages which slid freely upon horizontal steel guide bars some 

 4 cm in diameter and ground true. The two carriages were then 

 connected by an invar metal bar (fig. 4) to which they were 

 stoutly and permanently clamped. The whole system was 

 then free to move upon its guides, but the relative position of 

 the telescopes was fixed. The object of this arrangement was 

 obviously to secure a constant distance between the telescopes, 

 in spite of slight changes in the temperature of the system due 

 to the temperature of the room or the heat from the observer's 

 body, whatever the relative expansion of the various parts of 

 the apparatus. After a good many observations had been 

 made, it was found that the upright brass tubes supporting the 

 telescopes upon their carriages were not uniformly affected by 

 the heat from the body of the observer. They did not there- 

 fore expand uniformly and parallel to each other, but tended 

 to buckle very slightly during each series of observations. 

 This was subsequently corrected by a second invar bar above 

 the telescopes which in combination with the first formed a 

 rugged rectangular system which preserved the cross-hair dis- 

 tance without change throughout long series of observations. 



In mounting the furnace for observation, the side openings 

 which gave access to the scale divisions were directed down- 

 ward in order to reduce to a minimum the convection currents 

 of air which endanger the constancy of the temperature within. 

 The openings were also made as small as possible for -the same 

 reason. It therefore became something of a problem to bring 

 in light enough to illuminate the scale divisions and at the 

 same time to make observations of the change in length with 

 the temperature. The device adopted was this : In the opti- 

 cal axes of the telescopes and some 5 or 6 em beyond the 

 objective, small total reflecting prisms were mounted upon the 

 extended telescope tubes in such a way as to deflect the line of 

 sight at right angles and upward into the furnace. Above 

 these prisms and between them and the furnace (see fig. 5), 

 windows of plane optical glass were set at 45° in such a way 

 that they served to reflect the light from an incandescent lamp 

 upward from their outer surfaces without materially inter- 

 rupting the line of observation through the telescope and 

 total reflecting prism. By this device the path of the illumi- 

 nating light was the same as the path of the reflected light 

 which reached the observer, which served to give plenty of 

 illumination for the scale without increasing the size of the 

 openings beyond what was required to see the actual expansion 

 and to measure it. 



The illumination was provided by a single incandescent 

 lamp of 100 candle power with a spiral filament of stock type 



