ON THE FIXED STARS. 391 



first, second, and third magnitudes ; although it is true that they are not 

 by, any means placed at equal angular distances from each other. But 

 from a comparison of the light of different stars, we may infer, that if their 

 real magnitudes are nearly equal, their distances must increase much 

 faster than in this arithmetical progression ; that is, that the stars of the 

 second magnitude are more than twice as remote as those of the first, and 

 those of the third more than three times as remote. Mr. Michell found 

 the light of Sirius between 400 and 1000 times as great as that of a star of 

 the sixth magnitude ;* consequently, supposing these stars actually equal, 

 their distances must differ in the ratio of 1 to 20 or 30 ; since light always 

 diminishes in proportion to the square of the distance of the luminous ob- 

 ject. The light of stars of different magnitudes, situated near each other, 

 may ^Blf compared by viewing them through two apertures of different 

 sizes, cut in cards, one held before each eye, the apertures being reduced to 

 such magnitudes, that the stars may appear equally bright ; and the com- 

 parison may be extended to the light of the sun, by finding a star and a 

 planet of equal brightness, and calculating what proportion of the sun's 

 light must be reflected by the planet, upon the most probable supposition 

 respecting the disposition of its surface to reflect more or less of the light 

 which falls on it. 



The stars are in general dispersed without any regular order, but we may 

 observe in many parts of the heavens that a number of them are so much 

 nearer together than to the rest, as to form a cluster or nebula. The an- 

 cients had noticed some of the most conspicuous nebulae, but Huygens t 

 first directed the attention of modern astronomers to the large one situated 

 in the constellation Orion. Herschel^ has now given us catalogues of 2500 

 nebulae : many of them can be resolved by very high magnifying powers 

 into separate stars ; but others appear to consist of a luminous matter, 

 spread uniformly in the neighbourhood of the several stars to which they 

 seem to belong. (Plate XXXI. Fig. 455... 463.) 



It has been conjectured that all stars are disposed in nebulae, and that 

 those which appear to us to be more widely separated, are individual stars 

 of that particular nebula in which we are placed, and of which the mar- 

 ginal parts may be observed, in the form of a lucid zone, which is called 

 the milky way, being too distant to allow the single stars to be perceived 

 by the naked eye. This opinion was first suggested by Professor Kant, 

 the author of the system of metaphysics called the critical philosophy. 

 The idea was adopted by Lambert, who considers the largest stars as con- 

 stituting a distinct nebula placed among a multitude of others, which toge- 

 ther produce the appearance of a continued zone ; and Dr. Herschel has 

 investigated very particularly the figure of a single nebula, which would be 

 capable of being projected into the form of the milky way.|| We must not, 



* An Enquiry into the probable Parallax of the fixed Stars, Ph. Tr. 1767, Ivii. 

 234. 



f Systema Saturnium, p. 8. Ph. Tr. 1716, p. 390. 



J Ph. Tr. 1786, Ixxvi. 457 ; Ixxix. 212 ; 1802, p. 477. Catalogue of Nebulae in 

 Connaissance de Tems for 1783 and 1784. See also J. Herschel, Ph. Tr. 1833, &c. 



Photometria, 1139, 1140. || Ph. Tr. 1784, Ixxiv. 437. 



