HISTORY OF PHYSICAL ASTRONOMY.

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Gassendi is the next person who occupied his attention with the subjectof the irradiation of light. This philosopher entertained very erroneousviews respecting the physical cause of irradiation, but he made some inte-resting experiments for the purpose of ascertaining the laws by which itis regulated. In order to ascertain the effect produced upon the enlarge-ment, according as the field upon which the luminous object appeared be-came brighter, he measured the successive magnitudes of the moon's ap-parent diameter, as visible to the naked eye, from midnight till noon, andobtained the following results. At midnight the apparent diameter wasfound to be 38'; at dawn it was 36J-'; in clear daylight, 34I-'; afterthe sun arose, but before he had emerged above the vapours of thehorizon, 34^; at mid-day, when the sun shone with all his splendour,33'*.

Descartes , in his Dioptrics, published at Leyden in 1637, at length gavethe explanation of irradiation which is now generally received. He sup-posed that the extremities of the fibres of the optic nerve, though small,have some magnitude, and that in consequence, the impression producedby each pencil of rays which converged upon the retina, had a tendencyto propagate itself to a certain distance on all sides, although it would bestrongest at the point where the rays converged. When the light of theobject was feeble, the impression might practically be supposed to be con-fined to the central point; but in the case of very bright objects, the im-pact of the rays might be so intense as to make the lateral impressionvery sensible. It followed as an obvious consequence of this mode ofviewing the subject, that all luminous bodies would appear somewhatenlarged beyond their true apparent dimensions.

Newton, having discovered the unequal refrangibility of light, did notfail to perceive that the aberration arising from this cause alone, wouldhave the effect of vitiating all observations of the apparent magnitudesof bodies, as seen through telescopes. In fact the rays of different coloursbelonging to each pencil of light, instead of converging to a point by therefraction of the object-glass of the telescope, were so dispersed as toassume the appearance of a small coloured circle at the focus, and henceobviously would arise an enlargement of the image beyond its true magni-tude. Newton remarked that the enlargement arising from the unequalrefrangibility of light would be less according as the focal length of thetelescope was increased. In the Principia, he states that the apparentdiameter of Jupiter, at his mean distance from the earth, was found by theaid of Huyghens ’ telescope of 123 feet focal length to be 39"; while, on theother hand, its value deduced from the times of the satellites crossing theplanet’s disk, amounted only to 37^". He, therefore, concluded that thechromatic aberration of the telescope was somewhere about 2".f Hencein reference to the apparent diameter of Saturn, which was found bymeans of the same telescope to be 18", he remarks that “ if all falselight be rejected, the diameter of the planet will not remain greaterthan 16".” j

It is remarkable that Newton makes no allusion to irradiation in thecourse of his enquiries relative to the true apparent diameters of theplanets. It is difficult to say, whether his Silence arose from an absolute

* Opera Omnia, tom. iii., p. 395; Flor., 1727.

f Prin., lib. iii., Phenom. i.

J Prin., lib. iii., Phenom. ii.