38 PROCEEDINGS OF THE AMERICAN ACADEMY 



arsenic spectrum, as we first observed it, we did not think it advisable 

 to use the full power of the instrument. We therefore used five 

 prisms, as stated, and read to one minute of arc. We always began 

 each series of observations by setting the cross-wire of the micrometer 

 on the sodium line, after the telescope had been adjusted to the angle 

 of minimum deviation of this line as first observed. There was 

 seldom any observed difference in this angle. But when by change of 

 temperature, or otherwise, an alteration of two or three minutes had 

 taken place, we found, on readjusting the cross-wire, that the relative 

 position of the spectrum lines was, to the limit of accuracy of our 

 measurement, wholly unchanged. 



We give below the table of wave-lengths of the principal lines of 

 the arsenic spectrum. 



The wave-lengths printed in heavy type denote the bands which 

 are most brilliant and give character to the spectrum. The other 

 lines are less constant and less distinct, and in some instances may be 

 due to accidental causes. 



We were surprised to find among the bright lines, that the one 

 which in the table is enclosed in brackets corresponds to the green 

 thalium band, and upon examining the spectrum it appeared evident 

 that thalium must be present in the arsenic in large quantities, as the 

 thalium band was fully as bright as any of the arsenic bands. 



The accompanying diagram (Fig. 3 of plate) will give some idea 

 of the general appearance of the arsenic spectrum. 



