146 SCENERY OF THE HEAVENS. 



apparently every moment displacing the stars. The effect is not so sensible in the case of 

 the planets because of their disks. These, though small, are of sufficient magnitude to 

 bear without so much disturbance the minute agitations of the atmosphere, whereas the 

 stars, being only brilliant points, without any perceptible diameter, are completely 

 displaced. It has been observed, that in serene climates, and on the tops of high 

 mountains, the twinkling of the stars is much less powerful than when viewed from other 

 situations, which tends to confirm the preceding explanation as the true one. 



On directing the eye to the celestial vault, the impression made upon the mind is that 

 of an incalculable number of stars being visible that of the army of heaven consisting 

 of a host which our arithmetic will not suffice to reckon. It is well known that this is an 

 optical illusion. Their twinkling and disorderly position in the sky confuse and deceive 

 the sight. They are so scattered as not to be included at once in the field of vision. 

 Hence arises the idea, that the visible number, which is really very limited, is, on the 

 contrary, immense. This is the popular notion, and it is borne out by our first optical 

 impressions. But an ordinary eye will not be able to discern much above a thousand in 

 our firmament under the most favourable circumstances ; and including both hemispheres, 

 three thousand will be the outside number which a keen and experienced gaze will reach. 

 The Greek and Arabian astronomers distinguished some of the brightest stars by 

 particular names, which are recognised in our nomenclature of the heavens, as Sirius, 

 Aldebaran, Rigel, Arcturus, Capella, Canopus, and Fomalhaut, though a different mode 

 of proceeding is now adopted, having become necessary by the large additions made to 

 the ancient catalogues. To distinguish the stars in a constellation, the letters of the 

 Greek alphabet are now employed ; when these are exhausted, those of the Roman are 

 used ; and when these fail, numerals are resorted to. Thus a designates the most bril 

 liant star of a group, ft the next most conspicuous, y the third, and 2 the fourth. In this 

 way, the relative brightness of the members of particular constellations is indicated, not 

 that of the stars in general, for y Virginis is equal in brightness to a Aquarii. This 

 method was first introduced by John Bayer of Augsburg, in his " Uranometria, " in 1603; 

 who thus ranged the stars in the order of brilliancy, as they then appeared to the naked 

 eye, and it has been adopted by most succeeding astronomers. Referring to the whole host of 

 heaven, the stars are divided into classes, according to their apparent magnitudes, which 

 range from those of the first magnitude, or the brightest, down to the sixteenth ; but all 

 after the sixth are invisible to the naked eye, and are hence called telescopic objects. Of 

 the stars in both hemispheres, within reach of a practised gazer, which in round numbers 

 may be stated to be three thousand, the following are about the proportions belonging to 

 the different classes : 



First magnitude - - -20 



Second - - 70 



Third - - - -220 



Fourth magnitude 500 



Fifth - 690 



Sixth - - - 1500 



From direct experiments with the photometer, an instrument for measuring the 

 intensity of light, Sir "W. Herschel inferred as follows : 



Light of a star of the average first magnitude = 10O 



second = 25 



third = 12 



fourth = 6 



fifth = 2 



sixth = 1 



The intensity of the light, therefore, of a first class star, according to this estimate, is a 

 hundred times greater than that of one belonging to the sixth. Sir John Herschel has 

 arrived at a different conclusion, having found the light of Sirius, the brightest star in the 

 heavens, about 324 times that of an average star of the sixth magnitude. 



