378 Prof. C. A. Young's Preliminary Catalogue of the 



those which have been seen twice at least; a number observed 

 on one occasion (September 7) still await verification. 



The spectroscope employed is the same described in the 

 Journal of the Franklin Institute for November 1870; but cer- 

 tain important modifications have since been effected in the in- 

 strument. The telescope and collimator have each a focal length 

 of nearly 10 inches, and an aperture of J- of an inch. The 

 prism-train consists of five prisms (with refracting angles of 55°) 

 and two half-prisms. The light is sent twice through the whole 

 series by means of a prism of total reflection at the end of the 

 train, so that the dispersive power is that of twelve prisms. The 

 instrument distinctly divides the strong iron line at 1961 of 

 KirchhofFs scale, and separates B (not b) into its three compo- 

 nents. Of course it easily shows Q every thing that appears on the 

 spectrum-maps of Kirchhoff and Angstrom. The adjustment for 

 "the position of minimum deviation" is automatic; i. e. the 

 different portions of the spectrum are brought to the centre of 

 the field of view by a movement which at the same time also 

 adjusts the prisms. 



The telescope to which the spectroscope is attached is the new 

 equatorial recently mounted in the observatory of the College by 

 Alvan Clark and Sons. It is a very perfect specimen of the 

 admirable optical workmanship of this celebrated firm, and has 

 an aperture of 9^ inches, with a focal length of 12 feet. 



In the Table the first column contains simply the reference 

 number. An asterisk denotes that the line affected by it has no 

 well-marked corresponding dark line in the ordinary solar 

 spectrum. 



The second column gives the position of the line upon the 

 scale of KirchhofFs map, determined by direct comparison with 

 the map at the time of observation. In some cases an interro- 

 gation-mark is appended, which signifies not that the existence 

 of the line is doubtful, but only that its precise place could not 

 be determined, either because it fell in a shading of fine lines, 

 or because it could not be decided in the case of some close 

 double lines which of the two components was the bright one, 

 or, finally, because there were no well-marked dark lines near 

 enough to furnish the basis of reference for a perfectly accurate 

 determination. 



The third column gives the position of the line upon Ang- 

 strom's normal atlas of the solar spectrum. In this column an 

 occasional interrogation- mark denotes that there is some doubt 

 as to the precise point of Angstrom's scale corresponding to 

 KirchhofFs. There is considerable difference between the two 



o 



maps, owing to the omission of many faint lines by Angstrom, 

 and the want of the fine gradations of shading observed by 



