250 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ing was next turned about a vertical axis until the direct image of the 

 slit fell upon the plate. No lines below group C4 were revealed by this 

 jirocess, nor was the position of the lines greatly changed. It must be 

 noted, however, that the turning of the grating will somewhat distort the 

 spectra and change the value of /x to cm. 



The second grating to be investigated was of 21 ft. radius, 14,438 

 lines to the inch. It was arransred on Rowland's mountinj. The in- 

 tensity of the spectra given by this grating was very inferior to that 

 obtained with the 6 ft. instrument. The first spectrum was selected to 

 work with, the magnesium spark was used as a source. It was found 

 necessary to give an exposure of an hour, and to use a slit .2 mm. in 

 width. A good deal of care was taken to have the exposure of the 

 proper length ; if too short, the lines themselves would not appear ; if too 

 long, the lines would be obscured by fog. With this 21 ft. grating, a 

 very little fog would quite shield any very faint line. After some trouble, 

 the characteristic reproductions of group 2790 were found. They now 

 occurred, however, at ' positions corresponding to wave lengths of 

 1728 /x, 1079 /x. The widths of the groups were 4.8 mm., 2.8 mm., 

 1.8 mm. Thus, though these phantom lines have different apparent wave 

 lengths when measured on different gratings, their dispersion seemed 

 proportional to their wave length. It must be noted that with grating 

 No. 2 only the lines called Aj, Ci, and A4, C4 are visible, the other 

 fainter lines, as well as the fluted effect in the background, being totally 

 absent. This is well accounted for by the feebleness of the spectrum 

 obtained from grating No. 2. The apparent wave length of these groups 

 was obtained by the same method as that previously employed. Owing 

 to the great distance between the head group (2790) C and its reproduc- 

 tions, a cathetometer was used. The results are correct, however, to 

 better than 2 Angstrom units. The widths of the groups were measured 

 on the dividing engine and are correct to within .03 mm. The value of 

 1 cm. of plate length was obtained from measurements between two sun 

 lines. 



I feel certain that groups 2790 /x, Ci and C4 as obtained with grating 

 No. 2 are not in 1:2:3 ratio. 



The next grating investigated was one of 21,000 lines to the inch and 

 21 ft radius. Here the illumination was even more feeble than with 

 grating No. 2. Great trouble was experienced from fog. The lines in 

 magnesium, Ci and C!^, could not be found. The last grating examined 

 was similar to No. 1. It had a radius of 183 cm., 14,438 lines to the 

 inch ; it was prepared by Brashear ; it was ruled by Rowland's engine at 



