REVERSED AND NON-REVERSED SPECTRA. 



13 



In the following results the reflection from the mirror M, figure 5, was 

 used in the first order and from N in the second order, after leaving G'. Obser- 

 vations were made near N, figure 5. The displacement corresponding to 

 80 fringes was successively 0.0024, 0.0024, 0.0024, 0.0024 cm -> so that the 

 mean value 



io 6 5^ = 30.0 cm. 

 agrees with the above. 



Similar trial observations (combined first order from N and second order 

 from Af) were made with red light near the C line in series of 6 with a mean 

 value 5eXio 6 = 34.o cm. Again, near the b line (green) , giving 5eXio 6 = 27.2 

 and 27.8 cm. per fringe. These should therefore be distributed in terms of 

 wave-length, as in table 2. 



TABLE 2. 



and they are as nearly as may be expected in the ratio in question, seeing 

 that the total displacement for 60 fringes does not exceed 0.004 cm. For 

 accurate data it would be necessary to count many hundreds of fringes, and 

 to correct the de values by multiplying by sec (6 2 81) /2. I have not done 

 this, as the red and green fringes are not so distinctly seen as the yellow. 



4. Continued. First=order spectra. The apparatus was now readjusted in 

 such a way that first-order spectra were available from both mirrors. This 

 puts the grating G' at a greater distance from the line M and N than before, 

 for the angle 6 2 is smaller. A series of trial results were investigated in the 

 same manner as above, the mean values from 4 successive pairs of 80 fringes 

 each being, in three repetitions, 



o 6 = 29.o, 29.4, 29.5 cm. 



cm. 



o = 29.25 cm. 



somewhat smaller than half a wave-length of the D light used. Unfortu- 

 nately, the screw at 5 (fig. 4) here worked jerkily, to which the low value is 

 probably due. 



In this case sin 6i = \/Di, where Z?i = 352Xio- 6 cm. or 0'i = g38'; sin 0' 2 = 

 \/D z , where D 2 = 2ooXio- cm. or 0' 2 =i7V> whence 



< 7 = 2647 / and 5 = 73i' 

 In a later series of experiments, the play of the screw 5 was improved, so 



and from 5 pairs of too fringes each, 



This makes an average of 



