MR. W. lirGGINS ON THE SPECTEA OF SOME OF THE NEBrL^. 393 



more distant, takes place in the same proportion as the surface decreases in apparent 

 magnitude ; as long therefore as a distant object remains of sensible size in the tele- 

 scope, the object retains its original brightness unaltered. If, therefore, the light of 

 these nebulae be compared with a luminous body on the earth, we can obtain approxi- 

 mately the intrinsic intensity of their light in terms expressing a proportion to the ter- 

 restrial light selected for comparison. The values of the intrinsic brilliancy of the 

 nebulae obtained in this way must be smaller than the true amount, because they are 

 measures of the light after it has traversed an unknown extent of celestial space, and 

 has passed through the earth's atmosphere. The amount by which, from these causes, 

 the estimated brightness of the nebulse would be too small must remain for the present 

 unknown, since we have no data by which this loss could be even estimated*. 



Notwithstanding these errors of unknown amount with which the results of a com- 

 parison of the nebulae with a terrestrial source of light must stand affected, an attempt 

 to discover, even approximately, the intrinsic brightness of the gaseous nebulae has some 

 importance in connexion with the physical constitution which prismatic analysis has 

 shovm these objects to possess. The coincidence of two of the three lines forming the 

 spectra of some of the nebulae, severally with a line of hydrogen and the brightest line 

 of nitrogen, appears to indicate that they consist of aeriform matter. Highly transparent 

 bodies, such as these gases are, emit when heated but a feeble light compared with that 

 which would be radiated, at the same temperature, by more opake bodies. The invisi- 

 bility of these nebulae to the naked eye, though some of them are of considerable 

 apparent size, shows that they possess a very feeble degree of luminosityf . 



Besides these considerations, by means of similar photometric observations made at 

 considerable intervals of time, it would be possible to ascertain whether the intrinsic 

 brightness of the gaseous nebulae is undergoing increase or diminution, or is subject to 

 a periodic variation. 



• In 1744 CwksEkvx was led by theoretical speculations to assume that light is gradually extinguished in its 

 passage through space. By somewhat similar reasoning Olbers (in 1823) assumed that a star loses the -g-g^ 

 of its intensity in traversing a distance of space equal to that which separates Sirius from the sun. — Ober die 

 Durchsichtigkeit des Weltraums, Bode's Jahrbuch, 1826, s. 110-121. 



The elder Strute, from an examination of Sir W. Herschel's telescopic gauges of the Milky Way, supposed 

 that a star of the sLxth magnitude has lost —^ of its original intensity, a star of the ninth magnitude j^, and 

 the smallest star visible in Sir William Herschel's telescope 5^.— Etudes d'Astronomie Stellaire, p. 89. 



A fundamental element of the reasoning by which Struve obtained these values was, that the stars are distant 

 from our system in the inverse ratio of their apparent brightness. Since, however, the stars to which observation 

 assigns the largest parallax, 61 Cygni and a, Centauri, are less bright than other stars, in which no parallax, or 

 a much smaller one only, has been detected (not to refer to what may be regarded as an exceptional case, the 

 great inequality in magnitude of some binary stars), this assumption that the apparent brightness of stars 

 depends alone upon their distance, has been shown not to be true in the cases in which it has been confronted 

 •with observation. 



t " It is e-^ddent that the intrinsic splendour of their surfaces, if continuous, must be almost infinitely less 

 than that of the sun." — Sir John Herschel, Outlines of Astronomy, p. 646, 7th edit. 



