8 



KNOWLEDGE. 



[April, 1903. 



Tvpe IV. — Spectra with dark Hutings, due to carbon, 

 fading towards the blue end of the spectrum. All 

 stars of tliis type are fainter than 5th magnitude. 

 Includes Lockyer's Pisciajt group. 

 Tyj)e V. — Spectra consisting chiefly of bright Hues, some 

 of which are due to hydi-ogeu and helium, and others 

 are at present of unknown origin. On acct)unt of the 

 first examples having been discovered by Messrs. 

 Wolf and Rayet, they are often called " Wolf-Rayet 

 stars." 

 Stars of types O and I. are white, those of type II. vary 

 from yellowish-white to yellow, and those of types III. 

 and IV. range from yellow to red. 



An attempt to illustrate the more common spectral 

 types is made iu Fig. 5. The spectra are drawn as they 



eyepiece like an ordinary sun-cap), and having a cylindrical 

 lens, C, which comes between the eye and the prism. 

 Suppose that after centering a star in the usual manner in 

 the telescope the spectroscope is added without the 

 cylindrical lens. In place of the onlinary star image a 

 thin line will be seen, red at one end, and passing through 

 all the colours of the spectrum to violet at the other end. 

 Under these conditions the spectrum " lines " are mere 

 dots, and to make them more easily visible it is necessary 

 to broaden the spectrum by introducing the cylindrical 

 lens, so that the dots are spread out into lines. The use 

 of this spectroscope is thus very simple — the star is 

 centred and focussed iu the ordinary manner, and then 

 the spectroscope is slipped on the eyejjiece, no alteration 

 of focus being required. 



Hv 



He 



-Tlie Fire principal Types of Spectra. O. y Ononis ; I., Siriu: 

 lY., 152, Schjellerup. 



YELLOW. OEANGK. BED. 



; II., Arcturus ; III., a, Orionis; 



ajipear in prismatic spectroscopes, where the blue end is 

 more dispersed than the red. The spectra of the third 

 and fourth type stars are reproductions of drawings made 

 by Duner, who has made a special study of these stars. 



Rememliering that Fraunhofer made discoveries with a 

 telescope only a little over an inch in aperture, it will be 

 evident that the oliservation of the general features of the 



principal varieties of 

 spectra is possible 

 without any great 

 instrumental equip- 

 ment, though it must 

 be admitted that it re- 

 quires a considerable 

 amount of patience. 

 Still, a persevering 

 observer will be well 

 rewarded for his 

 trouble, even if his 

 telescope has no 

 greater aperture than 

 three inches, if he pro- 

 vides himself with a 

 suitable spectroscope. 

 One of the most convenient instruments for viewing the 

 spectra of stars is the Zolluer " ocular " spectroscope, the 

 parts of which are diagrammatically .shown iu Fig. 6. A 

 small direct-vision prism, P, is inserted iu a tube, T, 

 terminating in a flange, F (adapted to fit on the telescopic 



Fig. 6. — The Zollner Star Spectroscope. 



Another well-known arrangement is the McClean star 

 spectroscope. Here the eyliudrical lens is a concave, and 

 combines the functions of the cylindrical lens with those 

 of the telescopic eyepiece. The cylindrical lens alone, if 

 used as an eyepiece for the telescope, will show a star 

 image as a short line of light, of course without colour, 

 and this is spread out into a spectrum when the prism is 

 added. The procedure with this instrument is first to 

 centre the star in the telescope in the usual way, remove 

 the eyepiece, insert the spectroscope, and push in towards 

 the object-glass until the lines appear pjroperly focussed. 

 When once the focus has been determined it is useful to 

 mark the position of the draw-tube. 



Believing that a record of actual experience with small 

 instruments will be useful to many readers of Knowledge, 

 I have asked my friend, Mr. A. Foote, f.r.a.s., who has 

 devoted much attention to stellar spectra as observed with 

 a 3-inch telescope, to be kind enough to furnish me with a 

 few notes as to what he has been able to observe. In 

 acceding to my request, Mr. Foote writes : — 



" I am Terv glad to supply you with a few selected stars out of a 

 very large list wliich I have examined with small spectroscopes in 

 my 3-inch telescope. As you are aware, I prefer the Zollner form 

 of spectroscope, as I found the McClean more difficult to focus. I 

 also found that the McClean did not bring out the solar type stars 

 so well as the Zollner. 



"The stars which giie the easiest results to a beginner are those 

 of Type I., in which the hjdrogen lines are so prominent. In 

 SirtHs, the hydrogen lines at F and tt (U^ and H^) are easilv 

 picked up, and are rery broad and distinct. Vega is perhaps evena 



