HISTOBY OF PHYSICAL ASTBONOMY.

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order to cause one of the disks to differ in magnitude by one or even twotenths of an iuch *.

Dr. Robinson, in a paper communicated to the Astronomical Society in1831, gives an interesting account of some experiments which he madefor the purpose of ascertaining the amount of influence exercised by irra-diation on the apparent diameters of the sun and moon f. The methodhe employed consisted in observing lucid disks of different magnitudes,and measuring their apparent diameters when subjected to differentdegrees of illumination. A slip of brass with a small circular hole in itwas placed in the focus of an object-glass, and the hole being illuminatedby a lamp placed behind it, was then viewed through the object-glass bymeans of a telescope directed to it. By this contrivance the image of thehole seen in the telescope w r as rendered similar to that which would havebeen formed by a remote object like the sun or moon, since the raysof light transmitted through the hole emerged in parallel directionsfrom the object-glass. On looking through the telescope, the apertureassumed the appearance of a luminous disk about 17' in diameter ; but, oninterposing a slip of oiled paper between it and the lamp, it assumeda much duller appearance, resembling the aspect of the moon in a densefog. In this state Dr. Robinson brought the wires of the micrometer soas to be tangents to the disk; and then, having caused the oiled paper tobe removed, he instantly saw the opposite segments of the disk extendbeyond the wires. Bringing the latter again into a tangential positionwith respect to the disk, he determined the apparent diameter anew, theexcess of which over the previous measure manifestly gave him twice thebreadth of the annular enlargement arising from the increased brightnessof the disk. In this way he obtained a series of results, the mean ofwhich indicated an irradiation amounting to about 2".0. He repeated theexperiment with apertures of different apparent diameters, and obtained asimilar result in each case. Dr. Robinson was induced to conclude fromhis experiments that the main source of irradiation is in the eye; at thesame time he admitted that the aberration of the telescope might in somedegree contribute to the apparent enlargement of the object. The tele-scopes which he employed on this occasion did not appear to be affectedby any sensible aberration, from the distinctness with which they ex-hibited very close double stars, so that the enlargement indicated by hisexperiments must be considered as referable almost wholly to irradiation.

In one of his experiments it appeared to Dr. Robinson, that the bright-ness of the lucid disk was equal to that of the sun as seen in his transit.It might be expected, therefore, that the irradiation would be nearly thesame in both cases. The experiment gave him 2".8 for the differencebetween the least and greatest apparent diameters of the disk. Thisresult presents a satisfactory agreement with two values of the solarirradiation deduced from astronomical researches of a more recent datethan those of Lalande and Du Sejour. By a discussion of the transits ofVenus in the years 1761 and 1769, Encke obtained a value of the solardiameter which was less than that of the tables by 2".95 X- Wurm, bya similar discussion of the annular eclipse of 1820, deduced a result indi-cating that the solar diameter of the tables was too great by 3".32 §.

In one of his experiments, Dr. Robinson illuminated the field of view

* Phil. Trans., 1783, p. 1, et seq. + Mem. Ast. Soc., vol. v., p. 1, et seq.

J Mem. Ast. Soc., vol. v., p. 7. § Ibid., p. 8

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