Hutchins and Holden — Elements in the Sun. 453 



desired portion of solar spectrum photographed upon the up- 

 per half of the plate ; then the sunlight is excluded by a shut- 

 ter, and the image of the electric arc containing the proper 

 metal is allowed to fall upon the slit, and its spectrum photo- 

 graphed 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 ob- 

 tained, 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 disturbance of camera or plate. The me- 

 tallic spectrum, being thus photographed immediately below 

 the solar spectrum, can be compared with it at leisure. 



These spectra are then examined with the aid of a glass mag- 

 nifying about ten diameters, and an} r concidences between so- 

 lar and metallic lines carefully noted according to their wave- 

 lengths. In order to eliminate any personal error, they are ex- 

 amined by both observers separately, and their results after- 

 wards compared. 



To eliminate errors arising from suspected impurites of ma- 

 terials, as also from the impurities known to exist in the car- 

 bons employed, we took what we called " comparison photo- 

 graphs." For these, we placed in the carbon cup a portion of 

 the substances known or suspected to be present as impurities 

 in our metal, and then photographed the spectrum thus given 

 on the upper half of the plate; a piece of the metal under ex- 

 periment was then placed in the lamp, and the spectrum photo- 

 graphed on the lower part of the plate. Any lines due to im- 

 purities would then extend entirely across the plate, while those 

 of the pure metal would extend only half way. In addition to 

 this precaution we consulted all accessible tables and plates as 

 to the position of known lines of metallic spectra, and also corn- 

 pared together all our photographs of the same region. If all 

 of these tests left any doubt as to the origin of a given line, it 

 was at once subjected to special investigation until all doubt 

 was removed. 



The dispersion given by the apparatus in the order of spec- 

 trum in which we worked is such that a single wave-length oc- 

 cupies on the negative a space of 1*12 mm. This makes the 

 distance between the lines Y) 1 and D 2 6'7 mm., while the length 

 of spectrum from A to H is about 4*1 m. With so great dis- 

 persion it would hardly be possible to mistake the position of 

 a line by any very considerable amount, or to confound neigh- 

 boring lines belonging to different metals. 



