IIISTOKY OF PHYSICAL ASTRONOMY.
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disk always appeared too small, in consequence of the light of the visible-segment dilating in all directions and encroaching on the dark limb of themoon. As a simple mode of exhibiting the influence of irradiation, heremarked that if a ruler be held before the eye so as to intercept the lightof the moon, it will appear narrower at the part where it is projected uponthe lunar disk.
We owe also to Kepler an explanation of the physical cause of irradia-tion. He supposed that the rays of light from remote objects convergedbefore reaching the retina, and then proceeded in a divergent course, sothat each pencil of rays depicted upon the optic membrane a small sur-face instead of a point. It followed as an obvious consequence of thistheory, that the object would appear somewhat enlarged. Kepler, how-ever, was of opinion that unless the retina was very sensitive, no impres-sion was produced upon it beyond the central point of the small areacorresponding to each pencil of rays. Hence, according to this view of thesubject, the phenomena of irradiation would be visible only to a particularclass of individuals *.
The foregoing explanation is exceedingly plausible, but a slight examina-tion will suffice to show that it is inconsistent with the results of observation.In the first place, it is not true that the rays of light emanating from remoteobjects, converge before reaching the retina. The eye in fact possesses apower of adaptation which enables it to bring the rays to a focus upon theoptic membrane, however great may be the distance at which the object isplaced. Secondly, it is to be remarked that even if this assumption waswell founded, the hypothesis would still be inadequate to explain the phe-nomena of irradiation, since the latter do not fail to present themselves toobservation at the distance of distinct vision. Lastly, the phenomena ofirradiation are generally visible to all persons, although they exhibitthemselves with greater intensity to some persons than they do to others.
The irradiation of luminous bodies was a subject which attracted theattention of Galileo on many occasions in the course of his physical inves-tigations, and a variety of interesting remarks upon it are to be found in-terspersed throughout his writings. In the “ Sidereus Nuncius ” heasserts that the fixed stars, when viewed with the telescope, are not en-larged in the proportion of the magnifying power. In explanation of thisfact he remarks, that the telescope has the effect of stripping the star ofthe false light by which it is usually surrounded when viewed with thenaked eye. This spurious corona is ascribed by him to the influence ofirradiation. It generally increases with the brightness of the field uponwhich the luminous object is projected. Thus at sunset, when the ob-scurity of the heavens is tempered by the influence of the twilight, thestars, even of the first magnitude, appear excessively minute. So withrespect to Venus, notwithstanding the splendour with which she usuallyshines, she does not exceed a star of the sixth magnitude on those occa-sions when she happens to be visible at noon j-.
Galileo next alludes to irradiation in one of his letters to Welser on thesolar spots. Scheiner had remarked that Venus would appear as large onthe sun’s disk, as she does when she is near her superior conjunction.Galileo , however, denied the truth of this statement. He asserted, in thefirst place, that the irradiation of the planet usually causes her to appear
* Ad Vitellionem Paralipomena, p. 217, et seq.