^L. L. Day and J. K. Clement — Gas Thermometer. 435 



were removed to the colder parts of the furnace in order to 

 avoid this displacement, sufficient water vapor condensed upon 

 them from within to obscure the field, so that the window 

 scheme had to be entirely abandoned. 



The second considerable difficulty to be encountered was due 

 to the effect of the outside illumination of the divisions of the 

 bar in a field of rather high power (about 25 diameters). 

 Consider the bar to be illuminated by a beam of light from a 

 fixed source (which remains constant in position while the bar 

 expands) and the light received through the telescope into the 

 eye to be reflected from the polished parts of the bar surface 

 between the rulings. For reasons which appear in the adja- 

 cent figure (fig. 7), this reflected light does not show the lines to 

 be equally* displaced after expansion. The reason for this is 



Fig. 7. 



Fig. 7. Showing how the lines appeared displaced after expansion. Ac- 

 tual expansion, m to point indicated by the arrow. Apparent expansion, m 

 to u. 



plain after a brief consideration. If lines are ruled with a 

 sharp tool upon soft platinum metal which is afterward polished 

 to remove the burr left by the cutting tool, the effect is to 

 round off the two edges of each cut to a greater or less extent, 

 and thereby to present approximately cylindrical bounding sur- 

 faces to the incident light. The apparent boundary of the line 

 will then be defined by the reflection of this light from the 

 cylindrical surface into the telescope. Kow, if this cylinder 

 be moved laterally in the direction produced by the expansion, 

 the light will be reflected from a different point on the 



* The small expansions of the millimeter sections themselves have been 

 taken into account, although not explicitly mentioned in this discussion. 



