6qO philosophical transactions. [anno 178O. 



Sept. 18, 1779} the periodical star was visible to the naked eye, when I first 

 looked for it. October 6, 1779, the periodical star was exceedingly bright 

 this evening. It exceeded a, and (3 Ceti ; which latter is considerably larger than 

 the former, and affords a proof of the change in the magnitude of the fixed 

 stars; as we can hardly suppose Bayer should have made a mistake in the 

 magnitude of the first two stars of this constellation. The apparent magnitude 

 of Ceti was not round, but elliptical, when observed through the telescope, 

 and not very well defined; but as it was too near the horizon, this shape might 

 arise from that cause, for other low stars were also irregular in their forms, yet 

 Bellatrix was exceedingly fine and quite round. 



October 7, 17795 the periodical star was perfectly round in the telescopes, and 

 its apparent diameter well defined, full, and very large, for a star of that mag- 

 nitude. When I speak of the apparent diameter, I would be understood to 

 distinguish it not only from the real diameter, but also (if I may be allowed the 

 expression) from the real apparent diameter. To explain this a little more at 

 large: the body of the sun, for instance, is of a certain dimension, which we 

 call his real diameter, and this remains always the same. His apparent diameter 

 (which I here call real apparent) is changeable, according as we approach to, or 

 recede from him, and is between 3l' 33" and 32' 39"; but were he removed to 

 the distance of one of the nearest fixed stars, neither his real, nor real apparent 

 diameter, could then be known to us by any method we have hitherto been 

 acquainted with: for at the distance of at least 20 billions of miles, his real 

 apparent diameter could not much exceed 30"" of a degree; and a telescope 

 must magnify above 14000 times to make him appear of only 2 minutes in 

 diameter, which still is hardly sufficiently large to distinguish a square from a 

 circle: and yet I doubt not, but that we should observe some apparent diameter 

 or other of the sun thus removed from us; and this is what I here have called the 

 apparent diameter. This must be owing to some optical deception. De La 

 Lande explains it thus: " Si Ton voit dans les lunettes une lumiere eparse qui 

 environne les etoiles, qui les amplifie et les fait paroitre comme si elles avoient 5 a 

 6" de diametre, on doit attribuer cette apparence i la vivacite de leur lumiere, a 

 I'air environnant et illumine, a I'aberration des verres, a I'impression trop vive 

 qui se fait sur la retine." 



October 19, 1779? the periodical star preceded a very obscure telescopic star 

 at the distance of l' 45". 16, or 1' 50".47. At 12 o'clock, the periodical star is 

 now about the meridian, and brighter than a Arietis. 



October 30, 1779> the periodical star is still increased, and visibly larger than 

 (X Arietis, though not in the meridian at present. 



o'clock, Ceti being now in the meridian, is almost of a middle size 



between Aldebaran and « Arietis. Its apparent diameter by the telescope is also 



