A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men.

  • Nichol, J. P. (John Pringle), 1804-1859.
Date:
1860
Catalogue details

Licence: Public Domain Mark

Credit: A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men. Source: Wellcome Collection.

    779/952 (page 745)
    KEF titbe luminous circle below! (2.) The phenomena of interior conical re- fraction were verified and established as facts, with equal suc- cess : also by the care of Professor Lloyd. We may conclude in the woids of M. Plucker: No phy- sical experiment has made such an impres- of conical refraction Fig. 3. asion upon me as that XA ray of light entering a crystal, and issumg las a luminous cone, was a thing unheard otof, and without analogy. Sir W. Hamilton asnnounced the fact as a consequence of a form of tithe wave which he had deduced by long cal nculations from an abstract theory. * I confess I \Mt on the rack to learn if a result so extraor- idinary could be confirmed by experiment,—a re- •'.sult predicted by the theory recently erected by ;tthe genius of Fresnel. And now that Mr. Lloyd JJas demonstrated that the facts are in perfect ^correspondence with Sir Wilham Hamilton's ! Jxpectations, every prejudice against a Theory so . narvellously confirmed, ought surely to disap- oear. Refraction A.tniospheric. The mode in • Ivhich a ray of light is bent by refraction as it .oasses through our atmosphere, has been already laxplained generally under Dioptrics ; and Mi- '■ ■iXGE ^ refers to instances in which irregular >-ariations in the atmosphere have been known -0 impress upon the rays passing through them ■s'ery curious effects. The question, however, as 0 the regular or normal influence of our aerial nvelope on the direction of light, is of vastlv :-;reater consequence; for until we have obtained 1reply to it, we cannot assure ourselves how far h<he apparent position of any body, seen through f:.he atmosphere, is apart from its real position. ■ Jncertainty of this sort remaining, would evi- Iviently affect with serious error our conclusions ' -like as to the true places of the stars, and to the true altitude of any terrestrial object.—(1.) frke Theory of Astronomical Refractions, or the i- 'rmciples on the ground of which we must apply ■ correction to the observed places of the hea- ' --enly bodies has long attracted profound atten- '8 a most arduous problem—one that ■1 all lilcelihood will never be comi)letely solved, -he standing and intractable difficulty is this— >edo not know the exact constitution of the aimo- Vkere throuyh which the ray passes. It is satis- ictorily established indeed, that the rcfringent ower of common air is proportional to its dcn- 'ty, 80 tliat we can readily obtain a constant bat will enable us to deduce the amount of tliat ower for any ordinary density ; and fortunately he consideration of one very variable element of he atmosphere may be omitted, inasmuch as tlie -ariation of refringent power incident on the KEF presence of humidity, is almost entirely compen- sated by the concomitant variation of density. But what shall we say regarding the arrangement of the atmosphere in reference to onr globe's surface ? The answer nniversally given in these investigations, is in the form of an assumption, viz., that the atmosphere is arranged with its surfaces of equal density, spherical and concentric to the earth, and therefore that the differential of refraction is a function of the density and dis- tance from the centre. The inquirj- remains, What is the relation between these two variables, viz., the density of concentric strata, and their altitude or distance from the earth's centre ? This relation is utterly unknown, and the equation cannot be integrated unless it is known. As- sumptions are again resorted to ; sometimes a law is taken for granted, because of its supposed conformity yvith. known physical laws; some- times one is assumed, because of the facilities it affords for integration. Besides these uncertain- ties however, it may further be confidently as- serted, that the original or fundamental assump- tion is not true. Near the earth, surfaces of equal atmospheric temperature and density de- pend on the figure of the gromid ; varying even in contiguous localities, according to the structure of the soil—whether it be covered by forests, large bodies of water, &c., &c. Such irregulari- ties must extend at least as far as the region of the clouds—about six miles high; and this is the portion of the atmosphere which most power- fully acts on the ray of light. HappOy these and other disturbing causes produce no appre- ciable effect when the external object is near the zenith; and it is exceedingly fortunate, that through the multiplication of Observatories in different latitudes, so large a portion of the sky is now within neighbourhood of the zeniths of the best Instruments. Each Observatory has thus its peculiar and appropriate range of sky— a range within which its determinations must in all delicate inquiries be accounted of greatest weight. It must not be supposed, however, that analysts have succumbed under the embarrass- ments now enumerated. Smce the period of Newton—(who seems to have discerned the law that until we reach low altitudes the refraction is sensibly proportional to the tangent of the zenith- distance)—remarkable skill and industry have been expended on the construction of acceptable theories and convenient tables; and with almost unexpected success. The places of stars within 70° of the zenitli of an Observatory can now be corrected by aid of these tables, to a precision so great that the errors of single determinations due to incalculable irregularities, may reasonably be expected to be cliiiiiuatcd in the viean of a sufficient number; and corrections of tho places of objects considerably nearer the horizon may be much relied on, if they have beoa observed under favourable circumstances. {2'/.c reader will now turn to page CoO.}