APPARENT MAGNITUDES OF STARS. 



luminous object such, for example, as the sun has the same 

 apparent brightness at all distances ; and, consequently, that the 

 quantity of light which the eye of an observer receives from it 

 being in the exact ratio of the apparent area of its visible disc, is- 

 inversely as the square of its distance. It remains, however, to 

 explain how it can be that, after it ceases to have a disc of 

 sensible diameter, it does not cease to be visible. This arises 

 from the fact, that the luminous point constituting the image on 

 the retina, is intrinsically as bright as when that image has a 

 large and sensible magnitude. The eye is therefore sensible to 

 the light, though not sensible to the magnitude of the image ; 

 and it continues to be sensible to the light, until by increase of 

 distance the light which enters the pupil, and is collected on the 

 retina, though still as intense in its brilliancy as before, is so- 

 small in its quantity, that it is insufficient to produce sensation. 



31. Since it is certain that no body shining like a planet, with 

 borrowed light, could be visible at all, even with the aid of a 

 telescope, at distances far less than those which intervene 

 between the solar system and the nearest of the stars, it follows 

 that the stars must be self-shining bodies like our sun ; or, what 

 is the same, that our sun is only one individual unit of the vast 

 number of stars, which are scattered through the universe. This 

 being admitted, a question of much interest and importance 

 arises, to determine not merely the distance of our sun from sur- 

 rounding suns, and the distances of surrounding suns from each 

 other, but also the comparative magnitude or splendour of these 

 numerous suns, relatively to our own and to each other. 



32. One of the most essential data for the solution of this pro- 

 blem, is a sufficiently exact numerical estimate of the comparative 

 apparent lustre of the stars as they appear to the eye, relatively 

 to the sun and to each other. Yarious instruments have accord- 

 ingly been invented called Photometers or Astrometers, which 

 have attained this object with more or less precision. Without 

 entering into the details of the principle or construction of such 

 instruments, we may here state that by their means, the propor- 

 tion of the quantity of light transmitted to the eye by the sun or 

 moon, or by either of these objects, and a star, or by different 

 stars, compared together, can be ascertained. 



By such means, Sir J. Herschel compared the full moon with 

 certain fixed stars, and ascertained, by a mean of eleven observa- 

 tions, that its lustre bore to that of the star a Centauri, which he 

 selected as the standard star of the first magnitude, the ratio of 

 27408 to 1 ; in other words, he showed that a cluster consisting of 

 27408 stars equal in brightness to that of o Centauri would give 

 the same light as the full moon. 



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