42 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



magnesium, which is illustrated in Plate 1, the line of wave-length 

 2790.8,* which forms the most refrangible member of a characteristic 

 group, is seen to be reproduced four times. Two of these false spectra 

 are in the region between wave-lengths 1800 and 1900 and two in the 

 region between 900 and 1000. It is to be noted that of the two groups 

 near 1800 the less refrangible is the stronger, while of the groups near 

 900 the more refrangible is the stronger. This is in accord with the 

 theory, for if the two strong reproductions are considered to be the 47th 

 and 23d spectra of the grating of m groups, the two weak reproductions 

 are the 46th and 24th spectra, while the real line 2790.8 corresponds to 

 the 70th spectra. 



When, however, the wave-lengths as measured in the previous paper 

 were compared with those calculated from the fractions f-J X 2790.8, 

 $% X 2790.8 , f $ x 2790.8 , f § X 2790.8, the observed facts did not 

 seem to agree with the theory as accurately as might be expected. It 

 seemed worth while to remeasure the wave-lengths of the false spectra, 

 in order to determiue if the fault lay in the theory or in the observed 

 values. The author was encouraged in this step by the interest which 

 Professor Runge took in the matter. In fact the method employed in 

 re-measurement was suggested by him. 



The arrangement of the apparatus was as follows : — 

 Two slits, A and B, were placed upon the circle whose diameter was 

 the radius of the concave grating. The grating itself was kept fixed in 



position and the normal to its surface fell 

 midway between the two slits. By this 

 method the image of A was formed at B. 

 An arm carrying the plate-holder C was 

 pivoted at the centre of the circle. The 

 result of this plan was that the first spec- 

 trum obtained when A was used as source, 

 was shifted with respect to that obtained 

 with B as source by a definite amount. 

 The heights of the two slits were so arranged 

 that the A spectrum fell directly over the 

 B spectrum upon the photographic plate. 

 The method of procedure was to illuminate A with the light from a 

 magnesium spark and to place the plate-holder C in such a position that 

 a photograph of the false lines under investigation was obtained. Next 



* Exner and Haschek, K. Akad. der Wiss. in Wien, 106, Abth. II. (1897). 



