192 



Dr. J. R. Rydberg on a certain Asymmetry 



fault in the execution of the work, at least with regard to 

 the special grating in question. Hitherto, I had not deemed 

 it possible to make any of these assumptions, as it seemed that 

 Prof. Rowland himself and other spectroscopists who have 

 used the concave gratings ought to have recognized such an 

 anomaly, if it existed. 



During all the adjustments the grating was left in the 

 same position in its holder, so that I had made use only of 

 the spectra on one side of the grating. Now it was removed 

 from its holder and, after being reversed, it was adjusted in 

 the same manner as before, with the intention of learning 

 whether the focal curve that passes through the centre of 

 curvature is symmetrical with respect to the principal axis of 

 the concave mirror. Then it was found that the distance 

 between the grating and the eyepiece ought to be increased 

 in order to get distinct images, while before it was necessary 

 to diminish it. From this it was evident that the inaccuracy 

 in the position of the images was due to the grating. 



II. First of all the question was to determine the true 

 form of the focal curve that passes through the centre of 

 curvature of the grating. According to Prof. Rowland's 

 theory this ought to be a circle, which should have as a 

 diameter the straight line that unites the centre of curvature 

 with the apex of the grating. If the form of the curve differed 

 perceptibly from a circle, it would not be possible with these 

 gratings to obtain spectra of a uniform scale. 



Fig-, l. 



The form of the focal curve can be determined with the 

 greatest facility, if the apparatus is altered in such a manner 



