1' 



444 PHILOSOPHICAL TRANSACTIONS. [aNNO 1720. 



a little under the centre ; and on June 29 the sun being then in conjunction 

 with Sirius, it passed under the thread, so as to touch it with its upper edge. 

 Whence it appears, that in the space of the whole year, there had been no 

 other variation of the meridian altitude of Sirius, than the breadth of the 

 thread, which appeared equal to the diameter of the star, which he takes to be 

 5, or at most 6 seconds. 



Supposing this to be so, he then shows that the whole diameter of the annual 

 orbit is to the distance of Sirius as the sine of 6" to the sine of 39° 33' the 

 latitude of the star ; whence the aforesaid immense magnitude of its body is a 

 necessary consequence. 



But before this obtain a full assent, it may not perhaps be amiss to inquire 

 whether the supposed visible diameter of Sirius, were not an optic fallacy, oc- 

 casioned by the great contraction of the aperture of the object glass: for we all 

 know that the diameters of Aldebaran and Spica Virginis are so small, that 

 when they happen to im merge on the dark limb of the moon, they are so far 

 from losing their light gradually, as they must do were they of any sensible 

 magnitude, that they vanish at once with their utmost lustre ; and emerge 

 likewise in a moment, not small at first, but at once appear with their full light, 

 even though the emersion happen very near the cusp ; where, if they were 4* 

 in diameter, they would be many seconds of time in getting entirely separated 

 from the limb. But the contrary appears to all those that have observed the 

 occultations of these bright stars. And though Sirius be larger than either of 

 them, yet he is much less than two of them ; and consequently his diameter to 

 theirs is less than the square root of 2 to 1, or than 14 to 10; whence, in Mr. 

 Cassini's excellent 36-feet glass, those stars ought to be about 4" in diameter ; 

 and they would undoubtedly appear so, if viewed after the same manner ; where- 

 as we are aliunde certain, that they are less than one single second in diameter. 

 The great strength of their native light, forming the resemblance of a body, 

 when it is nothing else but the spissitude of their rays. 



As to the other part of the argument, that the alteration in the declination 

 of Sirius, on the score of the access of the earth in December, and its recess 

 in June, amounts to 6^' ; I can only remark, that, besides that a radius of 3 

 feet, as it seems that made use of was no more, is somewhat too small for so 

 extremely nice an observation, 6^ being subtended by the 1000th part of an 

 inch, some of the observations before recited plainly show, that the refraction 

 of the medium intermixed with those differences that might be occasioned by 

 the parallax. 



But the principal objection against the conclusion of this argument, seems to 

 be, that the meridian altitude of Sirius at Paris being under 25 degrees, the 



