Royal Astronomical Society. 459 



ROYAL ASTRONOMICAL SOCIETY. 

 [Continued from p. 225.] 



March 9, 1849.— On Irradiation. By Professor Powell. 



After adverting to the history of researches on this subject, the 

 author dwells particularly on the method of exhibiting the phseno- 

 menon adopted by M. Plateau, which forms the basis of all his own 

 experiments, and which consists of a card or lamina, cut so that one 

 half of a long parallelogram is cut out whilst the other remains, 

 having the portions at the sides cut away. Viewed against the 

 light, the enlargement of the bright half, in breadth, is seen con- 

 trasted with the opake, and might be subjected to measurement. 



The first question on the subject refers to the supposition of a 

 ij}QC\x\\BX physiological cause affecting the eye to produce the apparent 

 enlargement of the bright image. After fully allowing for some 

 portion of such phaenomena being fairly attributable to ocular causes, 

 such as dazzling, contrast, &c., experiments are adduced to show 

 that precisely similar phcenomena are produced in an artificial eye, or 

 camera obscura ; whence the hypothesis of any peculiar affection of 

 the retina is rendered unnecessary. The same conclusion is further 

 confirmed by photographic impressions of the image of the card cut 

 as before, which exhibit the same enlargement. Specimens of these 

 impressions, taken by Mr. N. S. Maskelyne, were exhibited. 



These results, clearly pointing to an optical cause, agree with the 

 conclusions of the undulatory theory, relative to the " diffraction of 

 a lens," as investigated by Mr. Airy, which apply to the eye con- 

 sidered as an optical instrument, as well as to the object-glasses of 

 telescopes ; in either case the image of a point being an extended disc, 

 which, if the light be bright enough, will be surrounded by rings. A 

 luminous surface will exhibit a like enlargement. 



Without reference to any theory, it is an ascertained law that the 

 enlargement increases with the intensity of the light. The enlargement 

 also is formed with a rapid decrease in brightness towards the edge. 

 On these grounds it is easy to explain the fact of the great diminu- 

 tion or total destruction of irradiation by the interposition of lenses, 

 which would follow immediately from the weakening of the intensity 

 in proportion to the square of the linear magnification. The author 

 has examined particularly into the extent to which this effect takes 

 place, and announces that low powers (from 5 to 20) are sufficient to 

 obliterate all irradiation even in the most intense light which the eye 

 can bear. 



Various results of M. Plateau and others as to the effects of con- 

 trast in making a narrow bar or wire continue visible, though the 

 irradiations ought to overlap, have been examined, and found only 

 to hold good with low intensities. 



The author next considers the effect in telescopes. Here that 

 portion of the effect which regards the ocular image being placed out 

 of consideration from the influence of the magnifying power (already 

 referred to), we have only to consider that part which affects the 

 focal image of the object-glass. The diminution of the aperture in- 



