MR. A. E. TTJTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 353 



e 



1 1* L ft - <i> ft - Q ft - <) ft - y 



^0 L ft I/ ft 2/ ft 'l) ft "!.) . *A> 



L_ T j /Jj JL*2 



Jjtj t/ti <pf]. 



Although the expressions 6 and </> appear long they are really very readily 

 computed from the experimental data, as there are only three sets of quantities 

 involved, the differences of the temperatures, the sums of the temperatures, and 

 the differences of the lengths at these temperatures ; also the two pairs of denomi- 

 nators are inversely identical. 



The results of the observations connected with the determination of the expansion 

 of the platinum-iridium tripod will now be given. Five independent series were 

 carried out, in order that this fundamental constant might be ascertained with the 

 utmost precision. With regard to the length of the screws of the tripod, given in 

 the tables of results, it should be remarked that the actual lengths of the three 

 screws are necessarily very slightly different, in order to produce the slightly wedge- 

 shaped air-layer required for the generation of the bands. The length given is 

 of course the mean length, and is the thickness of the air-layer at the centre, where 

 the passage of bands is observed. In order to determine this mean length of the 

 screws, besides direct measurement of each screw, which is somewhat difficult on 

 account of the more or less pointed character of the screw-ends and of the agate end 

 of the rod of the thickness measurer, recourse was had to the use of a plane parallel 

 disc of thick glass, whose thickness was accurately known, and which was laid 

 on the screws. 



Expansion of Platimim-iridium Tripod. 



SERIES 1. 



Length of screws above table, L, = 12*369 millims. 



Thickness of air-layer, d = 12*369 millims. 



Temperatures of observations, t, = 11*1 C., t, = 68*5, , = 117 0< 8. 



t i t l = 57*4. 3 ! = 106*7. 



Barometric pressures, b^ = 763*9, fc 2 = 765*2, & 3 = 766*4 millims. 

 Number of transited interference bands observed during the two intervals, 



/ 2 = 20*63,/ 3 = 39*40. 



Correction for alteration of refraction of air, + 2*17 and + 3*52. 

 Corrected number of bands, // = 22'80, / 3 ' = 42*92. 

 Wave-length of light employed (green mercury line), X = 0*0005460 millim. 



X == 0*000273 millim. 

 VOL. cxci. A. 2 z 



