406 G. Bar us — Methods in Reversed and 



Trial readings of the micrometer for a passage of 20 fringes 

 each were made without special precautions and showed 

 (omitting the data) an average of 10~° X 30l cm per fringe. As 

 the line of symmetry lay very near the two Z>,Z> 3 doublets, this 

 is obviously an approach to half a wave length. For accurate 

 work D,D^ and D\D\ should be superposed, in which case the 

 fringes would lie between and actually correspond to their 

 mean wave length. 



A number of measurements, like the above, were now made 

 with different types of fringes, and the ayerage values succes- 

 sively taken from 3 or 4 batches of 30 fringes each. 



The results were less decided when long fringes were used. 

 The final mean value of the 10 sets was he X 10" = 3019 cm per 

 fringe. Actual or approximate coincidence of the D lines 

 made no appreciable difference. 



In the following results the reflection from the mirror J/, 

 fig. 5, was used in the first order and from N in the second 

 order after leaving G' . Observations were made near ]¥, 

 fig. 5. The displacement corresponding to 80 fringes was 

 successively taken. 



The mean value 



10" 8e = 30-0 cm 



agrees with the above. 



Similar trial observations (combined first order from JV and 

 second order from M) were made with red light near the C line 

 in series of six with a mean value he X 10° = 34 , C '". 



Again near the b line (green) giving he X 10" = 27 - 5 cm per 

 fringe. These should therefore be distributed in terms of 

 wave length 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 ■004 cm . For accurate data it would be 

 necessary to count many hundreds of fringes, and to correct 

 the he values by multiplying by sec(# 3 — 0,) / 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 specrra were 

 available from both mirrors. This puts the grating G', fig. 3, 

 at a greater distance from the line M and N than before, for 

 the angle # 3 is smaller. A series of trial results were in- 

 vestigated in the same manner as above, the mean values from 

 four successive pairs of eighty fringes, each being taken in 

 three repetitions. They gave an average value of oe X 10° = 

 29"25 cm , somewhat smaller than half a wave length of the D 

 light used. Unfortunately the screw at s (fig. 4) here worked 

 jerkily, to which the low value is probably due. 



