94 PROCEEDINGS OP THE AMERICAN ACADEMY 



reproduce an exact copy of these with those of Kirchoff. It must, 

 however, be remembered that Kirchoff's and Srnythe's drawings, made 

 with prisms, will present a slightly different collocation from those 

 made with a grating. I give also both Kirchoff's and Angstrom's 

 drawings of B, that it may be seen how far justice has been done 

 heretofore to this interesting region in the best maps. In my own, 

 every line is given from repeated micrometric measurement. I have 

 only given part of the A group, therefore, which extends much fur- 

 ther, but in so faint a light that I have not felt sure of my measures 

 beyond A 12 . The relative intensities only are the result of estimate. 

 The scale is double Angstrom's. I have made as yet no studies below 

 the A group, but have discovered an unnoticed analogous group more 

 refrangible than B, which appears in the air-spectrum, and which I 

 do not give, as all these studies belong exclusively to the high sun. 

 From the successive appearances of these three groups, however, I 

 have drawn the inference that the spectrum below A, if ever rendered 

 visible, will probably present a strikingly different type from that 

 above B. 



As Captain Abney has lately succeeded in photographing as low 

 as A or lower, it may be proper for me to observe that the general 

 character of the observations here given in detail was, with drawings 

 of A and B, briefly presented by me to the notice of the National 

 Academy, in Washington, in 1877. I have executed this work with 

 apparatus partly due to the Rumford fund, and therefore have delayed 

 a full presentation till I could ask to be allowed the honor of sub- 

 mitting it to the American Academy. 



I have been assisted in these measurements by Mr. R. F. Hall and 

 Mr. F. W. Very. 



The spectroscope with which these observations have been made 

 is provided with two telescopes of 1.66 inch aperture and of 20.01 

 inches focal length, which are fixed in the walls of the cylindrical 

 chamber containing the diffraction grating. The angle between the 

 optical axes of the telescopes is fixed here, and it amounts to about 

 61° 16'. The grating is fastened to a revolving plate. A filar microm- 

 eter is attached to the observing telescope. The diffraction grating 

 used is of speculum metal, and is one of the largest made by Mr. 

 Rutherfurd, the actual size of the ruled portion being 1.75 inch square, 

 containing 17,296 lines to the linear inch. 



The object of these measurements was to obtain the relative dis- 

 tances between the lines, rather than their absolute wave-length. The 

 distances of the several components from the first line of each group 



