194 Dr. J. R. Rydberg on a certain Asymmetry 



spectrum and the movable carriage that bears the slit dis- 

 placed along the rail LC, until a distinct image was obtained 

 in 0. The position of an index, attached to the carriage, 



was read on a millimetre-scale fixed to the rail LC. Each 

 of the numbers given in the following table under a x and a 2 

 is the mean of 10 of these readings, the carriage being alter- 

 nately brought near to and removed from the grating. The 

 column «j corresponds to the spectra on one side of the 

 grating, the column a 2 to those on the other ; a greater 

 value denotes a greater distance from the grating. It was 

 found that the slit could be displaced through the space of 

 about one millimetre without it being possible to distinguish 

 any variation in the definition of the image. The probable 

 errors of the means amount in general to 0'2 millim., they 

 never exceed O'-i milliin. Using as a source of light some- 

 times the sun, sometimes the voltaic arc, I directed the 

 spectroscope in the spectra of the first four orders to the 

 weak lines between D ± and D 2 (A,:=5893) and to the double 

 lines b z and ?> 4 of the solar spectrum or the strong doublets 

 of the neighbouring band of carbon (X about 5165). 



Spectral 



Wave- 









Observed. 



Calcu- 

 lated. 



length. 



10 7 /*\. 



a v 



a 2 . 



IK+«2) 









line. 



d x . 



d.,. 



d. 



1.5 4 ... 



1x5165 



133-8 



153-8 



143-8 



-io-o 



4-10-0 



9-4 



I.D... 



1x5893 



1334 



155-6 



144-5 



-10-4 



4-118 



10-8 



II. 6 4 ... 



2 X5165 



123-2 



161-7 



142-5 



-20-6 



4-17-9 



189 



II. D... 



2x5893 



121-8 



165-2 



143-5 



-22 



4-21-4 



215 



III. 6 4 ... 



3x5165 



115-8 



171-6 



143-7 



-28-0 



4-27-8 



28-3 



III. D... 



3x5893 



1120 



1757 



143-8 



-318 



4-31-9 



32 3 



IV. & 4 ... 



4X5165 



106-0 



181-6 



143-8 



—378 



4-37-8 



377 



IV. D... 



4x5893 



1014 



187-6 



144-5 



-42-4 



4-43-8 



13-0 









M 



ean 143-8 









The first column contains the number of order of the spec- 

 trum and the line to which the spectroscope was directed ; 

 the second the approximate wave-length. In the fifth 

 column are the means of the values of a x and a 2 , which cor- 

 respond to the same line in the spectra on the two opposite 

 sides of the grating. These means approach, as we see, to 

 a constant value 143*8, which corresponds evidently to the 

 normal position of the slit at the vertex of the right angle 

 formed by the rails. On determining by direct measure- 

 ments this position, I have found 14.4 *8 + 0*1. But the 

 difference of one millimetre between the two numbers is 



