HISTOKY OF PHYSICAL ASTJ10N0MY.

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the object. It is different for different individuals, and for the sameindividual it varies from one day to another.

4° When an object is viewed through a telescope, the apparent enlarge-ment exhibited by it arises from two distinct causes, viz., ocular irradiation,and the aberration of the telescope. The part due to ocular irradiation de-pends on the magnifying power of the telescope, on the brightness of theimage, and on the physiological qualities of the eye of the observer. More-over, the interposition of the eye-glass of the telescope tends to produce apeculiar effect on the enlargement.

5° The part of the enlargement due to the aberration of the telescopevaries necessarily with the instrument employed. For the same instru-ment, it may be considered as constant.

The most remarkable of these propositions is that wherein'it is an-nounced that the irradiations of two luminous objects are diminished bytheir mutual proximity. M. Plateau extends this principle to the pheno-mena of irradiation seen with the naked eye as well as to those observedin the telescope. He rests its demonstration upon various experiments,some of which are of a very convincing nature, and do not seem to be liableto any objection. He refers to this principle the absence of any sensible irra-diation in the experiments of Arago, Eobinson, and Bessel, above alludedto. Without expressing a formal opinion on its origin he seems disposedto consider it as due to the fact, that there does not exist in either case asufficient contrast between the luminous object and the ground upon whichit appears projected. M. Plateau concurs with most other philosophersin placing the source of irradiation in the eye, but he does not express anyopinion respecting the mode by which the effect is produced.

The most recent account of experimental researches on Irradiation , iscontained in a paper communicated by Professor Powell to the Astronomi-cal Society, in the year 1849*. That distinguished philosopher appearsdisposed to regard the phenomena of irradiation as due, in a great degree,to some cause extraneous to the eye. While admitting that the irregularscintillation of a star may arise from some physiological affection of theorgan of vision, and may vary in different individuals, he considers thatthe enlargement exhibited by a well-defined disk, capable of exact mea-surement, cannot reasonably be ascribed to a cause of so fluctuating anature. He then proceeds to describe various experiments which tend to sup-port this assertion. One of the most unequivocal results derived from theseexperiments consisted in this, that an image of the object, formed in the focusof a lens, is affected by irradiation in an equal degree with the object itself.A card, similar to that employed by M. Plateau, being exposed to a mode-rately strong light, and its image being then thrown by reflexion upon aground glass placed in the focus of a lens, the image of the card was seenpainted on the ground glass, with precisely the same enlargement as thatwhich it exhibited to the naked eye. It might be urged against this ex-periment, that the brightness of the light at the focus of the lens might inits turn produce its irradiation, so as to occasion the apparent enlargementof the image. With reference to this objection Prof. Powell remarks, thatthe image on the ground glass was in general by far too faint to produce anysuch secondary effect. That the irradiation could not be due to this causewas proved further beyond all doubt by the fact, that when a dark glasswas interposed between the eye and the image of the card in the focus,

* Mem. Ast. Soc., vol. xviii., p. 69, et seq.