48 



PROPAGATION OF LIGHT. 



having any physical connection with it, at a 

 vast distance, and passes nearly in the form 

 of a great circle through the thick nebulosity 

 of Virgo (particularly in the northern w^ing), 

 through the Coma Berenices, the Great Bear, 

 the girdle of Andromeda, and the Northern 

 Fish. It probably intersects the starry milky 

 way in Cassiopea, and connects its poles, which 

 are poor in stars, made desolate by cluster- 

 forming forces, at the place where the stratum 

 of stars is of least thickness in space(^3). 



It follows, from these considerations, that 

 whilst our cluster of stars bears traces, in its 

 diverging branches, of greater transformations 

 effected in the lapse of time, and strives, through 

 secondary points of attraction, to resolve and 

 decompose itself, it is surrounded by two rings, 

 one vastly remote, made up of nebulae, and one 

 nearer, consisting of stars. The latter ring, 

 which forms our milky way, is a mixture of 

 unnebulous stars, on an average from the 10th 

 to the 11th magnitude('*), but, severally ob- 

 served, of very dissimilar magnitudes, whilst 

 isolated clusters of stars have almost always 

 the character of sameness. 



Wherever the vault of heaven is searched 

 with powerful space-penetrating telescopes, 

 stars, though perchance telescopic only, and 

 from the twentieth to the twenty-fourth in or- 

 der, or luminous nebulae, are discovered. Num- 

 bers of these nebulae will probably resolve 

 themselves into stars, when they come to be 

 examined with yet more powerful instruments. 

 Our retina receives the impression of single or 

 of thickly aggregated luminous points ; whence, 

 as Arago has lately shown, totally different 

 photometrical relations of the sensibility to 

 light result(*'). The cosmic nebulosity, form- 

 less or fashioned, generally diffused, producing 

 heat by condensation, probably modifies the 

 transparency of space, and lessens the equal 

 intensity of luminousness which, according to 

 Halley and others, must result, were every 

 point of the vault of heave"n beset with an end- 

 less succession of stars in the direction of its 

 depth(^^). The assumption of any such con- 

 tinuous inlaying of stars contradicts observa- 

 tion ; which, in fact, shows us vast starless 

 regions — openings in heaven, as William Her- 

 schel calls them — one in Scorpio, four degrees 

 in breadth, and another in the loin of Ophiucus ; 

 in the vicinity of both of which, and close to 

 their edges, we discover resolvable nebulae. 

 That which is situated on the western edge of 

 the opening in Scorpio, is one of the richest 

 and most thickly set clusters of small stars 

 that ornament the heavens. Herschel himself 

 ascribes the openings, the starless regions in 

 the sky, to the attraction and cluster-forming 

 force of these marginal groups(«'). " They are 

 portions of our star-stratum," says he, in the 

 fine liveliness of his style, " which have suffer- 

 ed great desolations from time." If we picture 

 to ourselves the telescopic stars that lie one 

 behind another, as forming a starry can'opy in- 

 vesting the whole of the visible vault of heaven, 

 then, I believe, are those starless regions of 

 the Scorpion and Serpent-bearer, to be regard- 

 ed as tubes, through which we see into the 

 farthest regions of space. The layers of the 

 canopy are interrupted ; other stars, indeed, 

 may lie within the gaps, but they are unattain- 



able to our instruments. The sight of fiery 

 meteors had already led the ancients to the 

 idea of clefts and chasms in the canopy of 

 heaven ; but these were regarded as passing 

 or temporary only. Instead of being dark, they 

 were luminous and fiery, by reason of the 

 translucent igneous ether that lay behind 

 them('^). Derham, and even Huyghens, appear 

 not indisposed to explain the mild light of neb- 

 ulae on some such grounds(®'). 



When we compare the brilliant, and on an 

 average certainly nearer, stars of the first mag- 

 nitude, with the telescopic or resolvable nebu- 

 lae, and contrast the nebulous stars with the 

 wholly unresolvable nebulae (with the one in 

 Andromeda, for example), or even with the so- 

 called planetary nebulae, in the contemplation 

 of distances so different, plunged, as it were, 

 in the boundlessness of space, we have a fact 

 revealed to us by the world of phenomena, and 

 the reality, which, in causal connection with it, 

 always forms its substrate — the fact of The 

 Propagation of Light. The rate of this prop- 

 agation, according to Struve's latest research- 

 es, is 41,518 geographical [166,072 English] 

 miles in a second ; nearly a million times 

 greater, therefore, than the rate of sound. 

 From what we know through the measure- 

 ments of Maclear, Bessel, and Struve, of the 

 parallaxes and distances of three fixed stars of 

 very unequal magnitudes — aCentauri,6lCygni, 

 and a Lyrae — a ray of light requires 3 years, 

 9i years, and 12 years, to reach us from these 

 celestial bodies severally. In the short but 

 remarkable period from 1572 to 1604, from Cor- 

 nelius Gemma and Tycho to Kepler, three new 

 stars blazed suddenly forth in Cassiopea, in 

 Cygnus, and in the foot of Ophiucus. The 

 same phenomenon showed itself in 1570 in the 

 constellation of the Fox ; but here it recurred 

 several times. In the very latest times, since 

 1837, Sir John Herschel during his sojourn at 

 the (3ape of Good Hope observed the star r] of 

 the constellation Argo increase in brilliancy 

 from a star of the second magnitude to one of 

 the first(»o). Such incidents in the universe 

 belong, however, in their historical reality, to 

 other times than those in which the phenome- 

 na of light notify their commencement to the 

 inhabitants of the earth ; they are the voices of 

 the past which reach us. It has been well 

 said, that with our mighty telescopes we pen- 

 etrate at once into space and into time. We 

 measure the former by the latter, the latter by 

 the former ; an hour of travel for the ray of 

 light is one hundred and forty-eight millions of 

 geographical miles passed through. Whilst the 

 dimensions of the universe are expressed in 

 the theogony of Hesiod by the fall of heavy 

 bodies — " the brazen anvil falls in no more than 

 nine days and nine nights from heaven to 

 earth" — Herschel, the Father ("), believed 

 " that the light of the farthest nebulae, which 

 his forty-feet reflector showed him, took about 

 two millions of years to reach the earth." 

 Much, therefore, has long disappeared, much 

 has already been otherwise arranged, before it 

 becomes visible to us. The aspect of the starry 

 heavens presents us with evidences of diversity 

 in point of time ; and diminish as we will the 

 millions or even thousands of years which 

 serve us as measures for the distance of the 



