138 THE INTERFEROMETRY OF 



an objection to the method, but no better device was found. Even so, the 

 windows frequently cracked and had to be replaced. Such an apparatus 

 naturally leaks, particularly at low temperatures, where the viscosity of air 

 is relatively small, so that the experiments as a whole are merely tentative. 

 To maintain the exhaustion as high as 70 cm., it was necessary to keep the 

 air-pump at work. To reduce this annoyance the exhaustions were at first 

 not carried above 60 cm. of mercury. With the interference fringes, however, 

 no serious difficulty was experienced after the tube had taken definite shape. 

 Distortion of fringes was inevitable, but centers of symmetry for measure- 

 ment were always available. 



The first experiments were made without exhaustion, at low and high 

 temperature (low red heat). The difference of displacement 8N between 

 cold (25) and hot was (for instance) in two different experiments 



25 loW =35.0 cm. 35.2 cm. 



red hot ioW =28.5 28.6 



or ioW= 6.5 6.6 



at atmospheric pressure. The 8 N so obtained makes no allowance for the 

 change of refractive index of the hot glass ends, nor for any displacement or 

 rotation or warping of the ends during the course of the experiment, which 

 required a lapse of an hour or two. 



In the next experiment, therefore, the method of exhaustion was attempted, 

 the partial vacuum used being about 16.6 cm. when the full barometer read 

 76.64 cm. Thus p = 6o cm. An example of the results obtained is given in 

 the following data. 



Cold Tube. 



Pressure 76.6 cm. ioW =34.8 cm. 34.7 cm. 

 Pressure 16.6 22.7 22.5 



p= 60.0 10^^=12.1 12.2 



Red-hot Tube. 



Pressure 76.6 cm. ioW =24.6 24.5 26.8 26.0 cm. 



Pressure 16.6 20.1 19.4 20.0 20.0 



p= 60. io 3 AA/" 4.5 5.1 6.8 6.0 



In the two experiments at the end readjustment was necessary, as the red-hot 

 tube warped during the exhaustion. In the last case the glass cracked. The 

 first two data should therefore be taken, so that 



io 3 AA/"=i2.i cm. io 8 A/\T'=4.8cm. io 3 5^=7.3 cm. / = 25 = 6 

 If equation (5) above is solved for t' the result is 



or if a =1/2 73 



/' 



This result is certainly small, as one would estimate the temperature (red 

 heat) at several hundred degrees higher. Unfortunately the relatively cold 



