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PROCEEDINGS OK THE AMEBICAN ACADEMY. 



tram. A dividing engine with a J mm. screw was used. This screw, 

 when used to measure lines on a six-foot grating possesses for a run of 

 20 cm. an ao uracy better than one one-hundredth of an Angstrom unit. 



y means of a stationary microscope fitted with 

 a four-inch objective and micrometer eye-piece. The value of one cen- 

 timeter of plate length in terms of Angstrom units was determined by 

 measurements takm between two given Bun lines, located l>v Rowland's 

 map. Iu order to get ;i photograph of the sun Bpectrum and the maf- 

 nesium spectrum on the same plate, a shutter was placed before the slit 

 and so adjusted that the upper or lower half could he exposed at plea- 

 By this means two narrow Bpectra were obtained, one above the 

 other. The method is preferable, when working with the firsl spectrum, 

 to the usual mode of protecting the plate itself by a swinging screen. 



When it became necessary t<> test the nature of the light 

 producing the lines al 1873 /u and '.•:_' 1/j. in the magnesium 

 Bpectrum, the following arrangement was adopted : The 

 quartz pri-.ni of an angle of five degrees was placed some 30 

 cm. distant from the slit, but not on the straight line joining 

 slit and grating. Thus when the source was in position A. 

 the light did not fall upon the prism ; but when it was in 

 position 15, the light passed through the prism. In the Iir.«t 

 case, the upper half of the slit was closed by the shutter; 

 in the second, the lower half was closed. The result was, 

 of course, to produce two spectra, one above the other; the 

 one due to light which had passed through the prism was 

 thus shifted toward the red. The general appearance may 

 be seen from Plate No. I. 



It now becomes necessary to consider more in detail the 

 behavior of the light of these ultra-violet Bpectra when 

 d through the quartz prism. Tin- results tor magne- 

 sium may be seen from the following table. The lettering 

 of the groups on the plans .,!' the spectra is, of course, 

 arbitrary. The chief or real groups are marked with an 

 Unsubscripted letter. The reproductions are marked with 

 letters bearing a subscript The plates are from drawings 

 made from the original photographs. Beginning with mag- 

 nesium Plate II., six principal groups of lines are to be 

 I: they extend from the pair of lines marked Aj, — 

 length 1966 /a, — to the Btrong group (',. — wave 

 length 924 /a. B< id< tl rong lines, there are six fainter groups 



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