Light / J.F.W. Herschel.

  • Herschel, John F. W. (John Frederick William), 1792-1871.
Date:
[1828?]
Catalogue details

Licence: Public Domain Mark

Credit: Light / J.F.W. Herschel. Source: Wellcome Collection.

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    Light. and w e cannot but believe that some other cause besides mere internal reflexions must inrterfere to prevent the Part !V complementary colour from reaching the eye; and that absorption, with its kindred phenomenon, or rather its «v— extreme case, opacity, is not satisfactorily accounted for in this theory, but must rather be admitted as (at pre- sent,) an ultimate fact, of which the cause is yet to seek. If this be granted, the colours of all bodies may be distinguished into true, viz., those which arise from rays 1143. whicJi have actually entered their substance and undergone their absorptive action, (as the colours of powders True and of transparent coloured media, cinnabar, red lead, Prussian blue, those of flowers, &c.,) and false, or superficial, false co- or those which originate obviously in the law' of interference ; thus, the variable colours of feathers, insects’ wings, striated surfaces, oxidated steel, and a variety of cases to which the Newtonian doctrine strictly applies, for there is no denying that cases of colour, not merely superficial, do occur, in which the Newtonian doctrine, to say the least, is highly probable. To instance one or two only. If a few drops of an extremely weak solution of Cases in nitrate of silver be added to a very dilute solution of hyposulphite of lime, a precipitate is formed of an opales- I'lchNew- cent whiteness and extreme tenuity. If more of the nitrate be added, the precipitate increases in weight and aggregation, and at the same time changes its colour, becoming first yellow, then yellow brown, then a rich orange brown, then a purplish brown, and, finally, a deep brown black. The precipitate, meanwhile, continually acquires density, and, finally, sinks rapidly to the bottom. It is impossible, in this series, not to trace the tints of the first order of reflected rings, produced by the thickening of the minute particles in the act of aggregation, but equally impossible not to recognise the agency of a cause totally diflerent, acting to increase the opacity of the compound by an absorptive action far superior to, and independent of, the action of the particles as thin plates. The phenomena of Hematine, described by Chevreul and cited by Dr. Brewster, {Encyc. Edin. Optics, p. 623; see also Biot, Trait'e de Phys. tom iv. p. 134, there referred to,) afford too close an approximation to the series of tints of the second order not to authorize a presumption that the Newtonian theory may apply to this case also. The diffused light and blue colour of the clear sky, affords another very satisfactory instance. This blue is, no doubt, a blue of the first order, reflected from minute aqueous particles in tlie air. The proof is, that at 74° distance from the sun, it is completely polarized in a plane passing through the sun’s centre. Another objection, no less obvious, to the Newtonian doctrine, has been successfully answered by Newton 1144, himself. A change of obliquity of incidence, it may be urged, should cause a change of colour, as a plate of Anoilierob- given thickness reflects a different tint at oblique and perpendicular incidences. But this variation is less, the jection. greater the refractive power of the medium ; and as the refractive power increases with the density, that of the Answered, ''dense ultimate atoms of bodies must be exceeding great, so that the tint reflected from them will vary little with i change of incidence, (art. 669.) Tlfe colours of oxidated steel afford an excellent case in point. The refractive power of this oxide, though great, (2.1), is, doubtless, not to be compared with that of the ultimate atoms of bodies, yet the tints on the surface of blued steel vary but little with a change of obliquity. We may add, too, that the colour exhibited by any body of sensible magnitude, is in reality an average of the colours reflected from all its molecules at all possible incidences, so that no change of incidence ought to be expected to affect it. Of the extreme tenuity of the ultimate molecules of bodies, Newton seems to have had but an inadequate 1145. idea, as he supposed that they might be seen through microscopes magnifying three or four thousand times.* Newton’s iVe have viewed an object without utter indistinctness, through a microscope by Amici, magnifying upwards of ideas of tt.e three thousand times in linear measure, and had no suspicion that the object seen was even approaching to size of the resolution into its primitive molecules. But it should rather seem that Newton regarded his colorific molecules ° as divisible groupes of atoms of a yet more delicate kind, and yet more densa, and these again as still further resolvable till the last stage of indivisibility be reached. M. Biot has given a striking, and, we may almost term it, picturesque account of this doctrine, in his Traite de Physique. § XV. Of the Calorific and Chemical Rays of the Solar Spectrum. It has long been a matter of everyday observation, that solar light exercises a peculiar influence in altering 1146. the colours of bodies exposed to it, either by deepening or discharging them, even when totally secluded from air, and that various metallic salts and oxides, especially those of silver, are speedily blackened and reduced when freely exposed to direct sunshine, or even to the ordinary light of a bright day. Whether these effects were owing to the heat of the rays, or to some other cause, remained long uninquired. The first step was The passage, however, is in the highest tone of a refined philosophy, and, independent of its theoretic bearings, we extract it, as indicating a scrutinizing spirit of observation far beyond the age he lived in. “ In these descriptions I have been the more particular, bec.ause it is not impossible but that m'croscopes may at length be improved to the discovery of the particles of bodies on which their coIou.rs depend, if they are not already in some measure arrived to that degree of perfection. For if those instruments are or can be so far improved as with sufficient distinctness to represent objects five or six hundred times bigger than at a foot distance they appear to our naked eyes, I should hope that we might be able to discover some of the greatest of those ccr- piisclfs. And by one that vs’ould magnify three or four thousand times perhaps they might all be discovered, but those which produce blackness. In the mean while I see nothing material in this discourse that may rationally be doubted of, excepting this position : That transparent corpuscles of the same thickness and density with a plate, do exhibit the same colour. And this 1 would have understood not without some latitude, as well because those corpuscles may be of irregular figures, and many rays must be obliquely incident on them, and so have a shorter way through them than the length of their diameters, as because thestraitness of the medium put in on all sides within such corpuscles may a little alter its motions or other qualities on which the reflection depends. But yet I cannot much suspect the last because I have observed of some small plates of Muscovy glass which were of an even thickness, that through a microscope they’ have appeared of the same colour at their edges and corners where the included medium was terminated, which they appeared of in other places. However, it will add much to our satisfaction, if those corpuscles can be discovered with microscopes ; which if we shall at length attain to I fear it will be the utmost improvement of this sensa. for it seems impossible to see the more secret and noble works of nature within the corpuscles, by reason of their transparency.”