182 osr an occttltatiojS or spica 



generally shine with a very steady light — my own observations in- 

 duce to the helief, that, Aldebaran is a star, of the 1st magnitude, 

 which does not shine with as steady a light as those I have named 

 above. Now with regard to the planetary appearance of the 

 stars on this occasion, irradiation cannot be overlooked. It is 

 irradiation which causes a luminous body when projected upon a dark 

 ground to appear of increased size, and the reverse of this holds 

 good, viz. : when a dark object is projected on a bright ground, the 

 light encroaches on the dark body, which consequently appears 

 diminished in size ; thus the new moon with the " old moon in her 

 arms," as it is technically expressed, shows this property of 

 irradiation very decisively, in the projection of the luminous portion 

 of the moon's disc, beyond the unilluminated part of the moon. 

 Irradiation causes Venus to appear circular when the planet is in 

 the form of a crescent ; I speak in these instances of unassisted 

 vision. The telescope under certain powers overcomes the effects 

 of irradiation, and strips both the stars and the moon of this false 

 light. In a transit of Venus or Mercuiy across the sun's disc, the 

 true diameters of the planets are diminished by the encroachmeut 

 of the luminous body of the sun. Irradiation, however, is diminished 

 by the illumination of the ground on which a luminous body is pro- 

 jected. Thus, stars of the 1st magnitude appear to the naked eye of an 

 inferior rank when seen in twilight or by moonlight ; and therefore 

 under ordinary conditions, the stars of the 1st and 2nd magnitudes 

 seen by me on the morning of the 13th March, should have exhibit- 

 ed less intensity of light, and presented less appearance of irradiation 

 in consequence of the bright moonlight, than they would have done 

 if seen on a dark night ; the reverse, however, was the case, for they 

 not only appeared with planetary discs, (if my vision is to be trust- 

 ed,) but also shone with greater brilliancy, and with a greater absence 

 of scintillation than ordinarily. To what causes are these appear- 

 ances to be ascribed ? 



It is well known that, when stars are viewed with the best telescopes 

 and with high magnifying powers, they present planetary discs, with 

 alternate dark and bright riugs surrounding them, but these discs 

 are spurious, caused it is supposed by the diffraction of light. View- 

 ed with low powers and in the finest instruments, stars of the 1st 

 magnitudes appear as mere points of light ; and that they have no 

 sensible discs, is proved by the instantaneous extinction of the light 

 of a star when occulted by the moon. As the night on this occasion 

 was frosty, and the moonlight strong ; the stars according to cus- 



