IRRADIATION, in physics, an apparent enlargement of a bright when seen against a background darker than itself. A simple method of observing this phenomenon is to view a bright sky through the spaces between a grating. If the breadth of the opening can be made equal to the breadth of the bars of the grating it will be seen that, when viewed from a little distance the bars look narrower than the spaces between them. This is obviously owing to the encroachment of the light upon the dark spaces around it. The first question to arise and one which was long discussed was whether the encroachment was due to an excitation of the nerves of the retina outside the limits on which the light fell upon the nerves, or whether it was necessary that the light should actually fall outside of its geometrical limits. The lat ter view is, found to be the correct one, unless in cases of extreme brilliancy of the light.
The phenomenon was explained by Plateau as due to the extension of the impression upon the nerves of the retina beyond the outlines of the image. Helmholtz, however, has ascribed it to the want of a perfect accommodation in the eye, leading to the formation of a diffiusion images about the proper image of a bright ob ject, so that it encroaches upon the dark space about it, and hence appears larger than it really is. Still another explanation is offered: Irradia tion is almost entirely in the nature of an optical defect or aberration of light. It begins with the atmosphere, which, when light passes through long stretches of it, slightly deflects the rays, so that a point is no longer seen as such, but as a small ill-defined waving surface. No
lens ever brings the rays from a point to ex actly the same focus. The lenses of the eye itself have defects which everyone who con, sults an oculist is acquainted with. The re sult of all these imperfections is to produce the enlargement we have described.
Irradiation is a notable subject in the history of astronomical observations. It was necessar ily larger with the imperfect telescopes of former times than with the improved ones of our own period. Total eclipses of the sun, the transits of Venus and Mercury were especially productive of the phenomenon. The enlarge ment of the moon resulted in a star appearing as if within the bright disc of the moon when its light was really only grazing the surface. The sharp points or horns of light formed by the limb of the sun during the transits of Venus and Mercury were rounded off, so as to present quite an illusory view of their form. Just at the beginning and end of total eclipses of the sun the phenomenon known as Bailey's beads, really enlargements of the last points of light from the suns limb, which could be seen before the sun was quite covered, looked like a string of beads. Many learned memoirs have been written on the subject, but the consensus of opinion to-day is toward the simple and comprehensive theories above mentioned. "Ir radiation* is also a name applied to the act of emitting beams of light, illumination, brightness emitted, and by extension, en-lightenment.