ASTRONOMICAL PHENOMENA AND PROGRESS. 



151 



respectively at 32.4, 24.8, 19.9, 16.8; the limit 

 at 14.5. In the spectrum of this last star no 

 fine lines have been found ; the lines observed 

 are all broad and black, with margins well de- 

 fined, being in fact so many complete interrup- 

 tions of the colored field. The spectrum of 

 a Orionis is marked by three broad bands, that 

 of Aldebaran by four, and that of /3 Pegasi by 



gives "Amer. Journal of Science," July, 1863) 

 an account of certain experiments intended to 

 determine on a new principle the relative in- 

 tensity of the light of the sun and fixed stars ; 

 and approximately, therefore, the relative dis- 

 tances of those bodies. 



Suppose a lens of known focal distance, say 

 one foot, is placed between the eye and a star of 



eight, these in all cases lying mainly within the jihe first magnitude, and that the lens is then 



s 



less refrangible half-length of the entire field ; 

 and all these, as well as the bands in the light 

 of Jupiter, are supposed by Mr. Rutherfurd to 

 be absorption bands due to the atmosphere 

 of the respective bodies, but which may yet 

 possibly be resolved into lines. 



In conclusion he alludes to the evidence now 

 possessed to the effect that the stars differ in 

 their constituent materials, and asks " "What 

 then becomes of that homogeneity of original 

 diffuse matter which is almost a logical neces- 

 ity of the nebular hypothesis ?" 



In his second article he mentions having add- 

 ed a prism by means of which the spectrum 

 from a spirit lamp is constantly present in the 

 field of view. He finds this a most useful 

 check, and by means of the comparison so af- 

 forded he has proved the presence in the spec- 

 trum of Arcturus of the lines D, E, b, and G, 

 and has become almost certain that each line 

 furnished by its light has its counterpart in the 

 solar spectrum. 



M. Secchi, of Rome, has used a Janssen's 

 spectrometer of direct vision, and he is aston- 

 ished at the magnificence of the results prob- 

 ably favored by an unusually pure atmosphere 

 which he thus obtains. He has published 

 the determinations only of five stars. He finds 

 in a Orionis a line at F, and four between F and 

 G, where one only is given in the Greenwich 

 observations. The spectrum of Aldebaran is 

 of greater extent, and 16 bands of various 

 breadths were noticed in it. He finds a spec- 

 trum of Rigel, as well as of Sirius, both white 

 stars ; these are longer than the spectra of red 

 stars, and in the former also the prominent 

 lines appear chiefly in the blue and violet 

 spaces at one extreme, and the red at the other. 

 The band F, which so far would appear to be 

 as prominent in the light of all the stars as it is 

 in that of the sun, Secchi thinks, may be due to 

 absorption by our atmosphere. 



Mr. Huggins and Dr. Miller have examined a 

 series of from 30 to 40 stars, and obtained mi- 

 croscopic photographs of Sirius and Capella. 

 The former takes the solar line D as the start- 

 ing point for his measures, having the solar and 

 a stellar spectrum in the field at the same time. 

 And since he finds that, generally, length of 

 spectrum corresponds with heat of flame, he 

 hopes that by means of these observations we 

 shall yet determine not only the chemical con- 

 stitution, but also the comparative heat of the 

 sun of our system, and certain of those other 

 suns which we name the fixed stars. 



The Sun and Stars Photometrically compared. 

 Mr. Alvan Clark, of Cambridgeport, Mass., 



made to recede from the eye in the line of the 

 star until the effect is to diminish the latter to 

 a point of light barely visible in other words, 

 to reduce it to a star of the sixth magnitude 

 at that moment when the lens (if convex) is 11 

 feet from % the eye : at this distance of the lens 

 the star has undergone a reduction of ten diam- 

 eters ; and accordingly, on the supposition that 

 there is no absorbing or extinguishing medium 

 in space, it follows that such star would itself 

 be visible, though then barely so, if it were re- 

 moved to ten times its distance in space from 

 the observer. Hence, the distance to which 

 different ones of the self-luminous celestial bod- 

 ies must be removed to bring them respectively 

 to the point of bare visibility to a minimum 

 visibile becomes a direct means of comparing 

 the intensity of their light ; and, so far as we 

 can assume their actual sizes and brilliancies 

 identical, a means of determining indirectly 

 their distances. 



In the use for this purpose of .a convex lens, 

 the measure is commenced at the focal point : 

 and the number of times the diameter is re- 

 duced is equal to the number of focal distances 

 the lens is removed less one (e. g., 11 focal dis- 

 tances, less 1, give a reduction of ten times in 

 diameter, and in brightness) ; but with a con- 

 cave lens, the measure is the actual distance 

 from the lens itself. For these observations 

 Mr. Clark has an underground, dark chamber, 

 230 feet long, accessible at one end from his 

 wockshop, and communicating with the surface 

 at the other end by a vertical opening, one foot 

 square. A common plane silvered-glass mir- 

 ror, set at a suitable inclination over the ori- 

 fice, reflects the rays of the sun or star down 

 the vertical opening in the ground upon a 

 prism so placed as to throw the light, by total 

 reflection, in the lino of the axis of the horizon- 

 tal chamber ; and no light can enter the latter 

 save through this lens. To the side of the 

 prism facing the chamber is cemented with Ca- 

 nada balsam (so perfectly as to render the two 

 optically one medium) the flat side of a plano- 

 convex lens, say of l-20th of an inch focus. 

 Then an observer in the cellar 230 feet distant 

 sees through this lens the sun reduced 55,200 

 times ; and its light varies little from that of 

 Sirius. To multiply the reducing effect, a sec- 

 ond lens of known focal distance, say 6 inches, 

 is mounted on a little car, which, by cords and 

 a pulley, can be sent to any required distance 

 on a track toward, and in the line of, the fixed 

 lens. 



At noon, March 19th, with a perfectly clear 

 sky, Mr. Clark found the sun barely visible 



