of the double refraction in strained glass. 489 



weighted mean was obtained, thus yW—y® was given double 

 weight and y® — y (0) triple weight, since the one represents two 

 and the other three fringe intervals. This gave a good determina- 

 tion of a mean fringe interval y {n+1) — y {n} in each case. The 

 result was then divided by the calculated wave-length. 



In calculating the latter a small correction has sometimes had 

 to be put in on account of the fact that the spectrum lines were 

 not, in all cases, perfectly straight, owing to spherical aberration 

 in the lenses. In most cases this correction was too small to be 

 applied. When sensible it was applied, so that the wave-lengths 

 given in the table below are corrected. 



The results obtained from the first two days' observations are 

 shown in the table (page 488). 



These show a steady increase of G from the red to the violet. 

 In other words Wertheim's law does not hold, but the relative 

 retardation in centimetres in air decreases as the wave-length 

 increases. 



The total variation is between 6 and 7 per cent, and could not, 

 it seems, be accounted for by errors of observation. The fringes 

 as seen by means of a powerful arc light were sharp and well 

 defined, easily measurable to tenths of a millimetre division. The 

 mean value of y {n+1) — y {n) should be accurate to the second place, 

 though even an error of one unit there would not account for the 

 difference. 



The experiments were repeated a large number of times, with 

 a view to eliminating the accidental errors. The results are ex- 

 hibited in the table (page 490). 



The sets taken on Feb. 24 and 25, those on Feb. 28 (II), of 

 March 5 and that of March 6 are from measures taken on the 

 fringes in the upper part of the beam (under tension). The 

 observations of March 1, 3, 4, 8, 10 and (1) of March 5 are made 

 on the fringes visible in the lower part of the beam (under com- 

 pression). 



The latter do not indicate such a large variation of C as do the 

 other observations. Those on March 1 and March 4 almost con- 

 firm Wertheim's law. But those of March 3 show a slight varia- 

 tion, tending to make C increase as \ diminishes ; those of March 5 

 and 8 show a strong variation and those of March 10 a moderate 

 variation in this sense. 



All the results corresponding to measurements taken upon 

 fringes in the upper half of the beam decidedly confirm this 

 variation. 



