OF ARTS AND SCIENCES. 293 



been, we should have been obliged to struggle with the question of 

 impurities in the carbons. An exposure of fifteen minu.tes to the ultra 

 violet spectra of metals burned in the electric light produced no image 

 below wave-length 3000. A quartz condensing lens was employed 

 with the arc light, and therefore no light was lost by selective absorp- 

 tion. With the spark no lens was necessary. 



By curving the photographic plate all parts of it remain in focus, 

 and distances on the plate are closely proportioned to wave-lengths. 

 Calling Y= wave-length, we have T= C + a x, where (7 and a are 

 constants, and x is the distance along the plate. 



The determination of the wave-lengths of lines extending over a 

 ranjie of three hundred tenth meters involved the taking: of three 

 negatives. The sensitive dry plate (2 X 10 inches) was pressed by 

 springs against the " forms " of the plate-holder into an arc of a circle. 

 Having placed the plate-holder on the camera box, the girder bearing 

 the camera and grating was moved along its tracks until the posi- 

 tion of the pointer of the carriage on the scale beside the track indi- 

 cated that light of wave-lengths 4200 to 4800 in the first spectrum 

 and 2100 to 2400 in the second spectrum would fall on the plate. 

 The shutter was turned so as to expose only the lower half of the 

 plate and a photograph of the solar spectrum from 4200 to 4800 

 taken. The shutter was again turned, and the upper half of the plate 

 given a long exposure to the light of the spark. Both spectra were in 

 focus. The wave-lengths of the metal lines were then found directly, 

 by interpolation on the normal spectrum, from the solar lines whose 

 values were given in Rowland's Photographic Map and table of wave- 

 lengths.* The interpolation was made by means of measurements on 

 a dividing engine. In order to correct for any displacement due to 

 the motion of the spark from side to side, or to jarring arising from the 

 great noise of the spark, and also in order to sift out the lines belong- 

 ing to the first spectrum from those belonging to the second, the girder 

 was moved to the violet of the third, with its magnified dispersion and 

 different underlying spectra. The metal and solar lines were taken 

 side by side, and the interpolation for the wave-lengths of the metal 

 lines made as before. From this the correction to be applied to the pre- 

 vious plate was found, amounting in some cases to .2 of a tenth meter. 

 The correction thus found was applied to all of the lines on the plate. 

 The girder was now moved so that the sensitive plate was in the ex- 

 treme ultra violet of the first spectrum, and the plate exposed to the 



* American Journal of Science, March, 1887. 



