412 G. Barus — Methods in Reversed and. 



These results are correct to -| per cent and are as close as the 

 estimation of p, c, &, and fractions of a fringe will warrant. 

 If results of precision were aimed at, a long tube should of 

 course be used. What was particularly marked in these ex- 

 periments was the motion of fringes in the passage from any 

 approximately adiabatic to isothermal conditions and on ap- 

 proaching a plenum of air. 



Since the refraction depends on density there should not, 

 apparently, be any motion at all ; but the thin tube is always 

 more nearly isothermal than the much larger barrel of the air 

 pump. As a consequence there is residual expansion from the 

 former to the latter. ■ 



D. The behavior of an old Babinet compensator, placed 

 nearly normal to one of the beams (see fig. 3), was peculiar, 

 though the fringes were clear and easily 

 Fig. 8. controlled. The dimensions of the right- 



handed quartz wedge were roughly cali- 

 pered and found to be : Length 4-2 cr ", thick- 

 ness at ends, 1-017 and 0-934 cm . Thus 

 there is a grade of -083/4-2 = -0193, or 

 something over 1° of arc. An up and down 

 displacement of 2 - 5 cm of this wedge was 

 available behind the stationary counteract- 

 ing left-handed wedge. 



The fringes were not uniform and they 

 required an inclination to the vertical of 

 the rulings of the grating G'. The fringes 

 were evidently curved lines, intersected by 

 the vertical strip within which they are 

 visible. Consequently they appeared as in 

 tig. 8, with linear elements in the middle, 

 shortening into dots at either end of the strip. On motion of 

 the compensator wedge, they moved toward or from the center 

 of symmetry, as is also indicated in the figure. Tiled fringes 

 were frequent. The most interesting feature however, was 

 their alternate appearance and evanescence, in cycles. While 

 the wedge was moved over 2-5 c,Q of its length, 7 of these cycles 

 appeared and vanished, each consisting of about 36 to 40 

 fringes. The disappearance was not always quite complete, 

 but the fringes could not be restored by any adjustment for 

 coincidence of spectra. 



An attempt was made to find the angle of the quartz wedge 

 by the first method. Data, -0023, -0024, -0024 em , were found 

 for the displacement of the micrometer per fringe. Hence 

 (apart from dispersion) 



10- 6 X 5-893 nnn ,. 



a = = -022 radians 



2 X -5442 X -0024 



which as in the glass plate is again above the calipered value. 



