1865.] Draper on American Contributions to Spectrum- Analysis. 399 



spectroscope thus mounted is useful for determining the achromaticity 

 of the telescope. If the different rays have foci at different distances 

 from the objective, the slit must be moved in order that each ray may 

 pass as a point. The place of the photographic focus may be thus 

 ascertained with precision. 



Mr. Rutherfurd examined the spectra of the Sun, Moon, Jupiter, 

 Mars, Capella, ft Geminoruni, a. Orionis, Aldebaran, y Leonis, Arcturus, 

 ft Pegasi, Sirius, Castor, a Lyrce, a Aquilse, Procyon, Regulus, ft 

 Ursse Majoris, K, UrssB Majoris, e Ursae Majoris, 8 Ursa? Majoris, 

 and printed a plate of them in ' Silliman's Journal,' vol. xxxv., May, 

 1863. a Virginis, ft Orionis, s Orionis, 5 Orionis, £ Orionis, a Ursse 

 Majoris, gave no lines. He found absorption bands in the spectrum 

 of Jupiter between C and D, and suggests the explanation that these, 

 together with the remarkable bands of a Orionis, Aldebaran, and ft 

 Pegasi, may be due to the action of the atmospheres of those bodies. 

 He classifies stellar spectra into three groups. 1st. Those resembling 

 the Sun, as Capella, ft Geminorum, a Orionis, Aldebaran, y Leonis, 

 Arcturus, /3 Pegasi ; all reddish or golden stars. 2nd. Those of 

 which Sirius is the type, wholly unlike the sun, and white. 3rd. 

 Those like a Virginis, white, but showing no lines ; they are perhaps 

 incandescent, without flame, or contain no mineral substances. A 

 great difference in the composition of stars is rendered evident. 



Another communication in the same volume commences by giving 

 a number of measurements of the companion of Sirius, and demon- 

 strates that it has changed 3° 37' in angular position, and 0*55" in 

 distance in a year, which would give a period of about 100 years 

 if the plane of motion is perpendicular to the line of sight. He 

 mentions a change made in his stellar spectroscope, by which a 

 sodium flame is kept constantly in the field. By the check thus 

 established, he proves that each line of the spectrum of Arcturus has 

 its counterpart in that of the sun. His large spectroscope is also 

 alluded to. The telescopes are 20 inches of focal length, and 1 r *L 

 inches aperture. It has six prisms of brass, with plane glass sides. He 

 finds that the bisulphide of carbon is very changeable in density, and 

 that the brass frames disturb the plane glasses when there are great 

 variations of temperature. On examining the line D, it is seen to be 

 composed of nine lines, a result subsequently corroborated by the use 

 of eleven prisms : the line B is resolved into fourteen. He also de- 

 scribes other peculiarities, particularly in the neighbourhood of A, 

 and in the potash spectrum. 



In a letter in No. CVL, Mr. Rutherfurd criticises the stellar 

 spectroscopes of Donati, Airy, and Secchi, giving the preference to 

 the last, because it effects the separation of the lines by the disper- 

 sion of its prisms, rather than by the power of the observing telescope, 

 and because it possesses the means of constant reference to a standard 

 flame. He remarks that in the line D, four of the nine component 

 lines are solar, the rest telluric, because these latter, though difficult 

 with eleven prisms at noonday, are seen with ease near sunset with 

 two. This conclusion is similar to that of Dr. Draper, concerning 

 the invisible lines a, ft, y, discovered by him below the red end of the 

 spectrum. 



