358 Mr. Louis Bell on the Absolute 



radiation should be at their maximum ; but the preceding 

 series, including as it does all the experimental errors, and 

 showing an extreme variation of but O'D leaves, I think, 

 little to be desired. 



The comparator was placed in a vault some six feet below 

 the level of the street, which was provided with thick double 

 walls with an air-space between. This observing-room enabled 

 the temperature to be kept down to a daily variation of less 

 than half a degree, the extreme range for several days being 

 frequently less than that amount. Before this vault in the 

 new Physical Laboratory was completed, the comparator had 

 been placed in an upper room of one of the old buildings, 

 where it was well nigh impossible to keep anything like a 

 constant temperature, particularly since the heat was unavoid- 

 ably partially shut off during the night, Owing to this state 

 of affairs the measurement of the gratings on which my pre- 

 liminary wave-length was based was made under difficulties, 

 and in most of the series necessarily under a rising tempera- 

 ture. Now when a glass standard is measured against a metal 

 one, glass, being a notoriously bad conductor, and having a 

 very small coefficient of expansion, if any rise of temperature 

 takes place the length found for the glass will be too small, for, 

 responding less readily to a change, it will be actually measured 

 at a lower temperature. 



It therefore became necessary to re-measure the glass 

 gratings Nos. I. and II., to eliminate this source of error, 

 which was done before the results for III. and IV. were 

 obtained. These gratings are very nearly 3 centim. long, and 

 they were therefore compared with successive triple centi- 

 metres of S a 2 until the fifteen-centimetre mark was reached. 

 Grating I. was first taken in hand and six complete series 

 of observations were obtained, each micrometer reading 

 being/the mean of several, and the extreme limits of tem- 

 perature-variation during the tw T o days occupied by the 

 comparisons being o, 3 C. The following gives a summary 

 of the results : — 



cm. 8. 



5G = 15 S* 



At 19°-9 C. 



+ 19-CH 

 : + 21-5 I 

 5G = 15S a 2 + 18-l ' 

 5G=15S a 2 + 23'9 f 

 5G= 15 S a 2 + 22-6 I 

 5G = 15S a 2 + 18-3J 

 Hence combining these and reducing them to the standard 

 temperature of 20° we have: — 



60,000 spaces=5G=15 centim. S a 2 + b"-2 at 20°. 



