438 PHILOSOPHICAL TRANSACTIONS. [aNHO ITQJ. 



Whatever then the real distance and magnitude of these stars may be, provided 

 we have not been greatly out in assigning the proportion of their light, in respect 

 to that of the sun and one another, if they were to be seen from a distance at 

 which the whole system would not subtend an angle of more than 6 or 8 minutes, 

 it would appear only as a nebulae, no single star being visible with perhaps any 

 telescope, that has ever yet been made ; for at this distance, the distance between 

 the earth and the largest star of this system, not subtending an angle of more than 

 about 3', that is, about a 1200th part of the radius, the stars of this system must 

 appear less luminous than they do to ourselves, in the proportion of the square 

 of 1 200, or 1 440000 to 1 . And supposing the light of Sirius to exceed that of the 

 least visible star, in the proportion of about 120O to 1, the brightest star therefore 

 would still require to have its light increased in the proportion of 1 200 to 1 , before it 

 would begin to be distinguishable. To do this, would require a telescope that 

 should take in a pencil of rays of a larger diameter than the pupil of the eye, in the 

 proportion of 35 to 1, that is, a pencil of about a foot diameter, exclusive of 

 deductions; for the pupil of the eye is not less than a third of an inch in 

 diameter, in a clear star-light night, when there is no moon ; but to obtain such 

 a pencil, we must not make use of a refracting telescope (with 2 lenses only) 

 of less than 15 inches, nor a reflector of less than nearly 2 feet aperture. 

 This may serve to show us, that those nebulae in which we cannot distinguish 

 any stars, may yet reasonably be supposed to consist of stars, though too far 

 distant to be singly visible; since this would be the case with our own system, 

 seen from as great a distance as we may well suppose those nebulae to be from 

 us, if we judge of it from the magnitude of the visible area which they occupy 

 in the heavens. 



Of the twinkling of the fixed Stars. 

 Having never yet seen any solution of the twinkling of the fixed stars, with 

 which he could rest satisfied,* Mr. M. offers the following, which may not per- 

 haps be found he thinks an inadequate cause of that appearance ; at least it has 

 undoubtedly some share in producing it, especially in the smaller stars. It is not, 

 he thinks, unreasonable to suppose that a single particle of light is sufficient to 

 make a sensible impression on the organs of sight. On this supposition, a very 

 few particles of light, arriving at the eye in a second of time, will be sufficient to 

 make an object visible, perhaps not more than 3 or 4 ; for though the impres- 

 sion may be considered as momentary, yet the perception, occasioned by it, is 



♦ Some astronomers have lately adopted, as a solution of this appearance, the extreme minute- 

 ness of the apparent diameters of the fixed stars, which, they suppose, must, in consequence of 

 this, be intercepted by every little mote that floats in the air ; but, that an object sliould be able to 

 intercept a star from us, it must be large enough to exceed the apparent diameter of the star by the 

 diameter of the pupil of the eye ; so that, if the star was a mathematical point, it must «tUl be 

 equal in size to the pupil of the eye.— Orig. 



