VOL. LVII.] PHILOSOPHICAL TRANSACTIONS. 427 



would scarcely be distinguishable with a telescope, on the former supposition, 

 that should magnify 6, or on the latter with one that should magnify 12000 

 times. 



Nor can we well expect to find their apparent diameters from any occultation 

 by the moon, since a diameter of the lOOth part of a second would be covered 

 by the moon, if it entered directly, in less than the lOth part of a second of 

 time, and therefore a star can hardly enter so obliquely, as to appear to vanish 

 by degrees; no star probably, which the moon can pass over, subtending an 

 angle half so great. A star might however appear to vanish by degrees in an 

 occultation by the planet Venus, especially if the occultation was to happen only 

 a little before or after either station; but this is an event which can occur so very 

 seldom, that little is to be expected from it; and if Venus should be surrounded 

 with an atmosphere, which is probably the case, it might very possibly then be 

 of lit) service at all. For the same reasons also it is probable, that nothing can 

 be determined from occultations by any of the rest of the planets, which on other 

 accounts are still less proper for the purpose than Venus. 



There seems to be little chance therefore of discovering with certainty the real 

 size of any of the fixed stars, and we must consequently be contented to deduce 

 it from their parallax, if that should ever be found, and the quantity of light 

 which they afford us, compared with that of the sun. And in the mean time, 

 till this parallax can be found, or something else may arise to furnish us with a 

 more general analogy, we can only suppose them, at a medium, to be equal in 

 size to the sun, this being the best means, which we have at present, of forming 

 some probable conjecture concerning the extent of the visible universe. That 

 we may be the better enabled to do this, it seems to be an object worth the at- 

 tention of astronomers, to inquire into the exact quantity of light which each 

 star afibrds us separately, when compared with the sun; that, instead of distri- 

 buting them, as has hitherto been done, into a few ill defined classes, they may 

 be ranked with precision, both according to their respective brightness, and the 

 exact degree of it. 



A catalogue of the stars formed on this plan, would answer several good pur- 

 poses; fo/ besides giving us a more just and certain notion of their general dis- 

 tances, it would perhaps help us, especially if the parallax of a few among them 

 should be discovered likewise, to trace some analogies in their situation, which 

 might enable us to determine both their distances, and other circumstances re- 

 lating to them, with still more probability; and it would be a standing register, 

 by which future astronomers might inform themselves of many variations, of 

 which we are now ignorant for want of an ancient register of that kind. 



From what has been said above, Mr. M. thinks it is very probable that we 

 shall not be a great way from the truth in estimating the whole parallax of Sirius 



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