452 Hutchins and Holden — Elements in the Sun. 



placed at our disposal by Professor John Trowbridge, under 

 whose supervision and directions the subsequent work has 

 been done. 



After some delay caused by the mounting of the grating and 

 its attachments, work was begun early in January, 1887, but, 

 owing to bad weather and other hindrances, was not regularly 

 and systematically prosecuted till somewhat later. 



The grating used is of speculum metal with a ruled surface 

 measuring 6 inches by 2, having 14,438 lines to the inch. It 

 is concave, its radius of curvature being 21-g- feet, and is mounted 

 according to Professor Rowland's method. Suffice it to say, 

 that the method is such that, by simply rolling the camera 

 along an iron track, it passes not only from one part of the 

 spectrum to another, but also to the spectra of different orders, 

 at the will of the operator. As the distances on this track are 

 proportional to the relative wave-lengths of the lines that fall 

 successively on a given point in. the camera, it is easy, by means 

 -of a suitable scale of equal parts placed beside the track, to set 

 the center of the photographic plate instantly within a single 

 wave-length of any given line in the spectrum. 



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 identified upon the map, and its absolute 

 wave-length thus determined. Although some of the nega- 

 tives 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 light is brought into the room by means of a porle 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 •§• 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 con- 

 taining 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 lautern. In the lower carbon of the lamp is made 

 a cup-shaped cavity, which is filled with the substance a spec- 

 trum 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 



