MB. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 361 



the approach of the two reflecting surfaces, a slight recession was found to occur. 

 Moreover, as the metallic tripod was affected more rapidly by the rising temperature 

 of the air bath, a recession for several bands was first observed, due to the expansion 

 of the screws ; the movement of the bands subsequently became arrested for a con- 

 siderable interval of time, until, when the badly conducting glass eventually 

 approached the same temperature as the tripod, one or two of the transited bands 

 retraced their steps past the spider-lines, so that the actual number of bands 

 eventually found to have effected their permanent transit was very small, corres- 

 ponding to close similarity of expansion between the glass and the tripod. 

 The results obtained were as follows : 



Expansion of Black Glass. 

 SERIES 1. 



Thickness of black glass block, L^ = 12'956. 

 Length of screws above table points, / 13'120. 

 Thickness of air-layer, d = 0*164. 

 Temperatures, t, = 8'9, t, = 68'9, t, - 122'6. 



t., ti= GO'O. t s , = 1137. 



Pressures, 6, = 739% & 2 = 739'2, b a = 739'4. 



Number of transited bands, f, = 2'83,f a = 4'66. 



Correction for air refraction, + 0'03, + 0'05. 



Corrected number of bands, f> = 2'86,f 3 ' = 471. 



Wave-length of light employed, X = 0-0006562, X = 0-0003281. 



Increase of thickness of air-layer, // X = "00093838. /,'^X - 0'0015454. 



Elongation of tripod screws, 



10- 9 8600 + 4-56 



10- 9 f8600 + 4-56 



ti + 



(t z - ,) = 0-0069092, 

 (t a t^ = 0-013275. 



Expansion of black glass block, 



L* 2 - L, = 0-0069092 G'00093838 = 0'00597082. 



L<, IA = 0-013275 0-0015454 = 0'0117296. 

 ^ = 0-000094226. (f> = O'OOO 000 067 939. 

 L = 12-95514. 

 a = O'OOO 007 273 3. b = O'OOO 000 005 244 2. 



VOL. CXCI. A. 



