HISTORY OF l’HYSJCAL ASTRONOMY.

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of the complete correction of any star in the catalogue for any day of theyear, and the consequent ascertainment of its apparent place on the sameday, is an operation -which may be performed in two or three minutes.The advantage which the theoretical astronomer must possess, in beingthus relieved at the outset of his researches from a vast amount oftedious calculation, is too obvious to require further notice. The valuesof the four constants depending on the time of observation, are calcu-lated for each day of the year, and are published annually in the Nau-tical Almanac * * * § . Mr. Airy has recently proposed a modification ofBessel’s method, designed to obviate certain inconveniences which insome respects attend its practical application j-.

The Uranographical principles, to which allusion has already been madein this chapter, exercise a direct influence on the apparent positions of thecelestial bodies, and therefore an accurate knowledge of the effects pro-duced by them is indispensable in all those questions which concern therelations of space and time. Moreover, the principle of refraction, byaffecting the aspect of phenomena, demands attention in various researchesrelating to celestial physics.

But the principles of Diffraction and Irradiation are also known to ex-ercise an influence similar to that ascribed to refraction in the latterinstance. These, indeed, by occasioning an alteration in the apparentmagnitudes of the celestial bodies might be supposed in some cases ulti-mately to affect their apparent positions. Their influence in this respect,however, has been hitherto found to be inappreciable. On the other handit cannot be doubted, that they sensibly affect the aspect of eclipses bothof the sun and moon, as well as transits, occultations, and varions otherphenomena. A brief historical account of researches on these principles,considered only in so far as their influence on astronomical science is con-cerned, may, perhaps, not be out of place on the present occasion.

The discovery of the Diffraction of Light is due to Grimaldi, an Italianphilosopher of the seventeenth century J. Having admitted a beam of lightthrough a very small aperture into a dark room, he found that theshadows of bodies placed in the cone of light thus formed, when receivedupon a screen, were larger than they would have been if the rays of lighthad passed by the edges of the bodies in right lines. He perceived alsoaround the shadows three fringes of coloured light. The fringe nearestthe shadow in each case was the broadest; the one most distant fromit was the narrowest and faintest of the three. Grimaldi concluded fromthese facts that light was deflected when it passed by the edges of bodies.He first gave an account of his experiments in a work published atBologna, in the year 1665 §.

The phenomena of diffraction did not escape the attention of Newton.In the third book of his Optics he gives some account of his researches onthe subject. He mentions, however, that he was interrupted in the midstof his experiments, and that at no subsequent period could he find leisureto resume them. This part of the researches of the illustrious philo-sopher must therefore be considered as left in an unfinished state.

Newton repeated Grimaldi’s experiment, by admitting a beam of light

* These are the constants denominated A, E, c, D, the numerical values of which are in-serted in succession at page xx. of each month, in the first part of the Ephemeris .

+ Mem. Ast. Soc., vol. xvi., p. 256, et seq.

| This property of light is also sometimes distinguished by the term “ Inflexion.”

§ Physico Mathesis de Lumine. Bonon, 1665.