HISTORY OF PHYSICAL ASTRONOMY.

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indication of either of these phenomena during the transit of the planetabove referred to. But he obtained a still more unequivocal proof thatthe effect of irradiation was insensible. Since the first interior contact ofthe two bodies is indicated by the sudden closing together of the twoextremities of the luminous border of the sun, while the second interiorcontact is made apparent by the equally sudden rupture of the oppositeborder, it is manifest that the interval of time included between the twocontacts when taken in conjunction with the relative motion of the planet,as assigned by the tables, will serve to determine the apparent diameterof the sun. Now Bessel, by employing this method, arrived at a re-sult which coincided exactly with that obtained by direct measurement ofthe solar diameter with a micrometer during the transit of the planet.

Thus it appears that the experiments of Bessel concur with those of M.Arago and Dr. Robinson in indicating that the effect of irradiation is in-sensible when the instrument employed in measuring it is one which givesa double image of the luminous body. We shall presently have occasionto mention that experiments of this nature have been supposed to furnishonly particular illustrations of a general law affecting the irradiation oftwo luminous objects placed in juxtaposition.

In the year 1838 M. Plateau communicated to the Boyal Academy ofBrussels , an elaborate memoir, containing an account of a variety of ex-periments performed by him with the view of elucidating the nature andlaws of irradiation*. A very simple mode of exhibiting the enlargementarising from this cause was devised by him. A white card was dividedinto six rectangular compartments, by drawing two parallel lines prettyclose to each other down the middle of the card, and then bisecting theseby a third line, drawn at right angles to them. The middle rectangle inthe upper half of the card, and the broad lateral rectangles in the lower half,were then painted black, so that there appeared in the middle of the carda band composed of two parts, the one black, (in the upper half of thecard,) projected upon a white ground, the other white, (in the lower half,)projected upon a black ground. This contrivance manifestly tended torender the effect of irradiation more perceptible ; for while the white handin the lower half of the card was liable to dilate from the encroachment ofthe light upon the black ground, the black band in the upper half wasequally liable to contract from the encroaching irradiation of the whitelight on each side of it. The card being placed vertically near a window,so as to he well exposed to the light, was then viewed at the distance ofa few yards, when the effect of irradiation was clearly exhibited by thewhite band in the middle of the lower half appearing sensibly broaderthan the black band above it. The following are the more importantconclusions to which M. Plateau was conducted by his experimental re-searches on the subject of irradiation.

1° The quantity of irradiation increases with the brightness of the ob-ject, hut in a much less rapid proportion. It has very nearly attained itsmaximum when the brightness is equal to that exhibited by the northernregion of the sky.

2° Two irradiations in close proximity tend to neutralize each other. Thediminution arising from this cause is greater in each case, as the intervalbetween the luminous objects is less.

3° The quantity of irradiation augments with the time of contemplating

* Nouv. Mem. de l’Acad. Royal de Bruxelles, tome xi., p. 1, et seq.

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