REVERSED AND NON-REVERSED SPECTRA. 



67 



The corresponding displacement found was = 0.92 cm. The computed dis- 

 placement should be = 0.80 cm. The low value was supposed to be due to 

 the need of readjustment (wedge-shape) and insufficient normality of plate. 

 Further data (table 13) were therefore investigated for thinner plates, but 

 they do not clear away the difficulties. 



TABLE 13. Inverted spectra. = 9 39'. x = 2e cos (90 0)/2. 

 = 1.526; 5 = 4.6X10-"; 25/X 2 = 0.0265 ; z = E ( M - l) +2ES/X 2 . 



In table 13, e is the observed normal displacement of the mirror M, x the 

 corresponding computed path-difference, z the path-difference computed for 

 the glass of thickness E and the index of refraction ju. From z the displace- 

 ment e or from x the glass thickness E may be computed. These are given 

 in the table. In the second and third parts of the table, the edge of the prism 

 P' was inclined at an angle to the line of symmetry, in opposite directions.. 

 The effect of this is manifest, but it does not explain the very large values of" 

 x for the symmetrical adjustment (rays from grating to mirror making an; 

 isosceles triangle) of the next observations. These were made with care. P',. 

 figure 43, lay at the middle of the base of the isosceles triangle of rays from 

 G. The long collimator was used, giving two splendid spectra, and rays were 

 raised until magnificent large, cord-like fringes appeared. The longitudinal 

 fringe, a single line normal to the sodium lines extending throughout the 

 spectrum, made it possible to set the micrometer to about 0.0003 cm - r 5 

 wave-lengths. The motion of fringes with the rotation of plate being very 

 striking, the plate was placed in the normal position to the rays by noting 

 the retrogression of fringes at this point. The total range of displacement 

 between extreme hair-like striations was 0.25 to 0.30 cm. One circumstance 

 was noticed for the first time that on limiting the blade-like beam from 

 below (as above described) the fringes not only enlarge, but rotate i.e., 

 path-difference is modified. 



It is thus difficult to ascertain the discrepancies in x in table 13, as these 

 values should be nearly 0.41 cm. throughout, or in e, which should be about 

 0.27 cm. when the prism is symmetrically placed. 



32. Inverted spectra. Further measurements. The curious results shown 

 in table 13 induced me to endeavor to detect the nature of the discrepancy 

 by displacing the prism in the direction of the beams incident upon it. To 



