292 PROCEEDINGS OP THE AMERICAN ACADEMY 



the aid of the new Map of the Solar Spectrum published by Professor 

 Rowland, it is very easy to determine the wave-length of metallic lines 

 in the visible spectra of metals ; for it is merely necessary to photo- 

 graph a portion of the solar spectrum upon the same plate as that 

 which receives the spectra of the metals under consideration, and then 

 to refer to the published map. We have already remarked, that even 

 a superficial examination of hitherto published catalogues of wave- 

 lengths of metallic spectra will show that distinguished observers differ 

 in their determinations by one or two wave-lengths. The task of re- 

 measuring the wave-lengths of metallic lines is a very great one, and 

 approaches in character the routine work now prosecuted in astro- 

 nomical observatories in the redetermination of star places, the photo- 

 metric intensities of stars, and the classification of star spectra. In 

 our present work we have confined our attention to ultra violet spec- 

 tra. Since the solar spectrum disappears in the neighborhood of wave- 

 length 2800, the task of identification of wave-lengths becomes a very 

 serious one. To replace the solar spectrum we must refer the lines 

 of metallic spectra to carefully measured lines of certain metals. When 

 one metal ceases to give spectral lines, another must be selected. To 

 test the relative accuracy of what we have termed the old method of 

 measurement with that of the new, we have measured the lines of 

 electrolytic copper, and have compared our results with those of pre- 

 vious observers in regard to the distribution of errors. Besides the 

 comparison of accuracy, we have examined the limit of the spectra of 

 copper in the ultra violet, in order to see if that given by previous 

 observers could be extended. 



Apparatus. 



The apparatus consisted of a concave grating of 21 ft. 6 in. radius, 

 mounted in the manner described by Professor Rowland. The camera 

 was provided with a shutter, which enabled us to expose different 

 portions of the sensitive plate at pleasure. An alterating dynamo 

 machine was employed, together with a Ruhmkorf coil. The alter- 

 ating machine gave from eight to ten thousand reversals per second. 

 With a battery of from six to ten two-quart Leyden jars, a powerful 

 spark was obtained between the metallic terminals which we employed. 

 The spark was produced close to the slit of the apparatus, and the 

 time of exposure varied from one to two hours. At various times 

 endeavors were made to substitute the more powerful light of the 

 carbon electric light for the electric spark, in the hope of shortening the 

 time of exposure ; but these efforts were not successful. If they had 



