HISTORY OF PHYSICAL ASTRONOMY.

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the enlargement still continued to be equally perceptible. Prof. Powell,upon the grounds of such experiments, considers the conclusion to be un-avoidable, that the enlargement is occasioned by some optical cause actingupon the formation of the focal image, and independent of any organicaffection of the eye.

Pursuing this view of the origin of irradiation, he conceived that a strik-ing verification of it would be obtained, if a photographic image of the ob-ject could be formed, exhibiting the enlargement. An experiment which hemade with a view to decide this point, was attended with complete success.The image of the card when formed by a photographic process, either inthe light of the bright sky or in the full sun, was found to exhibit a mani-fest enlargement.

Irradiation may be greatly diminished, or even completely destroyed,by the interposition of a lens between the object and the eye. Prof.Powell found that with ordinary daylight a lens which magnified three orfour times was sufficient to extinguish every trace of the phenomenon.He states, as the result of his experience generally, that magnifying powersvarying from ten to twenty effectually destroy the irradiation occasionedby the brightest light which the eye can bear.

M. Plateau experienced great difficulty in reconciling the effect producedby the interposition of a lens with the theory of irradiation, which as-cribes its origin to some ocular cause. Prof. Powell is of opinion that itmay be accounted for by the diffusion of light, resulting from the ap-plication of the lens, rendering the enlargement too faint to be percep-tible. Upon the same principle he explains the absence of irradiation inexperiments made with a double image micrometer. In this case, each ofthe images possesses only half the original brightness, and being at thesame time viewed with a high magnifying power, the effect of irradiationmight, by the combined operation of both these causes, be rendered in-sensible.

In observations with the telescope it has been found, that the regularenlargement occasioned by irradiation is mainly dependent on the aperture,focal length, and magnifying power of the instrument. With respect tothe new moon, Prof. Powell remarks, that when viewed with a 30-inchachromatic, and very low powers, the projection of the illuminated crescentbeyond the dark part of the disk was distinctly seen; with a power of 50 itwas barely visible; and with 80 not at all. The following passage of a letterfrom Prof. C. Piazzi Smyth, cited by that philosopher, affords an interesting-confirmation of the same fact:—“ On a particularly fine night at the Cape,when the enlargement of the bright part of the new moon beyond the darkwas unusually striking to the naked eye, amounting to two or threeminutes of space, a 14-foot reflector was turned to the object, and, withevery increase of power, the apparent projection was more and more cutdown—definition was very good that night. With power of 300 the pro-jection was barely sensible; it did, however, still absolutely exist, but toperhaps not more than two or three tenths of a second of space.” *

It is generally admitted that the irradiation of bodies is affected by thedimensions of the aperture of the telescope. When the aperture is dimin-ished, the apparent enlargement of the object is diminished also. This,doubtless, arises from the less intense brightness of the image. It is tobe remarked, however, that when the aperture is reduced to very' small

Mem. Ast. Soc., vol. xviiii., p. 80.