OF ARTS AND SCIENCES. 15 



And here let us parenthetically state that all Our wave-lengths are 

 those given by Professor Rowland's photographic map of the solar 

 spectrum, the position of every line referred to being carefully identi- 

 fied upon the map, and its absolute wave-length thus determined. 

 Although some of the negatives contain many lines too faint to show 

 on the map, yet we feel confident that our numbers correspond in all 

 cases to those of the map within one tenth of a wave-length. 



The lhdit is brought into the room by means of a forte lumiere, 

 and then sent through the slit after total reflection by a right-angled 

 prism. Before striking the prism it passes through a cylindrical lens, 

 which condenses it to a band of light about 2 inches long and I inch 

 wide. The jaws of the slit move equally in opposite directions, so 

 that, however widely they may be opened, no lateral displacement of 

 lines can result from this cause. 



Directly in front of the slit is placed a large tin lantern containing 

 an electric lamp ; the image of the arc can be brought exactly upon 

 the slit by means of an adjustable lens in the front of the lantern. 

 In the lower carbon of the lamp is made a cup-shaped cavity, which is 

 filled with the substance a spectrum of which is desired. It is not at 

 all necessary that this be in the form of a metal, for any ordinary 

 compound is at once reduced by the intense heat and the presence 

 of carbon vapor to the metallic state. 



The plan of working has been as follows. The apparatus being 

 arranged as described, the sunlight is admitted and the desired portion 

 of solar spectrum photographed upon the upper half of the plate ; 

 then the sunlight is excluded by a shutter, and the image of the elec- 

 tric arc containing the proper metal is allowed to fall upon the slit, 

 and its spectrum photographed on the lower half of the plate. (Most 

 of the plates used were those made by the M. A. Seed Co., and were 

 cut to the size of 8 inches by 2. The most sensitive plates were 

 obtained, and even then we found the required time of exposure for 

 some parts of the spectrum inconveniently long.) 



In order to effect the exposure of either half of the plate at will, 

 we placed directly in front of the camera an opaque screen, in which 

 was a rectangular opening one half the size of the plate. By turning 

 a handle, this screen is raised or lowered without the slightest disturb- 

 ance of camera or plate. The metallic spectrum, being thus photo- 

 graphed immediately below the solar spectrum, can be compared with 

 it at leisure. 



These spectra are then examined with the aid of a glass magnifying 

 about ten diameters, and any coincidences between solar and metallic 



