THE CAUSE OF THE ZODIACAL LIGHT. Scientific American.

Hawke's Bay Times, Issue 1622, 23 October 1874, Page 403

THE CAUSE OF THE ZODIACAL LIGHT. Scientific American.

Professor Arthur W. Wright, of Ynle College, communicates to the American Journal of Sciencp and Arts a valuable paper on " The Polarisation of the Zodiacal Light," in which the experiments of the investigator are detailed, and results given which will probably set at rest the moot question as the nature of that celestial phenomenon. The zodiacal light is a faint nebulous radiance which, at certain seasons of the year, and especially within the tropics, is seen at the west after'twilight is ended, or in the east before it has begun. The luminosity is conical, in shape, the breath of the base varying from Bdeg. to 30deg. in angular magnitude, and the apex being sometimes more than 90dcg. in rear of or in advance of the sun. To account for this appearance several theories have been advanced. Oassini believed it a lenticular solar emanation: Kepler considered it the Bii n's atmosphere, and Maeran a reflection from the latter stretched out into a flattened spheroid. Laplace declared the phenomenon to be a nebulous rotating ring, situated somewhere between the orbits of Venus and Mercury, and Chaplain Jones, U.S.N., whose examinations into the subject have been the most extensive on record, also believed it a nebulous ring, and continuous, and not located as stated by Laplace. Professor Wright's deductions, as will be seen, fail to agree exactly with any of these views. But few attempts, it appears, have ever been made to determine whether or not any portion of the light is polarised, and up to the present time knowledge on the subject has been uncertain and contradictory, pointing either to the idea that the rays are not polarised at all, or that the proportion of polarised light is so small as to render it nearly impossible to be detected. Professor Wright, becoming convinced that the difficulty should be •ascribed to the imperfections of the instruments employed, constructed a new apparatus, 'consisting of a quartz plate, cut perpendicularly to the axis, and exhibiting by polarised light an unusual intensity of color. It is a made, the body of the plate consisting of lefthanded quartz, through which passes eccentrically a band of right-handed quartz, bounded by two intermediate j strips of different structure. Placed between two Nicols, these strips appeared as bands of color, upon dark or light ground) according to the turning of the prisms. This plate, mounted in a tube with a Nicol, formed a polariscope of extraor- ; diuary sonsibility, and the first favorable opportunity to test its powers on the ■ zodiacal light was improved. It was almost immediately fouud to indicate the existence of light polarised in a plane passing through the sun ; and in no instance, when the sky was clear enough to render the bands visible, did their position, as determined by the observation, fail to agree with what would be required by polarisation in the plane above noted. Not the slightest trace of bands was ever seen when the instrument was directed to other portions of the sky. The observations took place on clear, cold nights, when the moon was absent. The polarisation, it was also proved, did not arise from faint vestiges of twilight, the reflection of the zodiacal light itself in the atmosphere, qr from impurities in the latter. Further experimenting was at once proceeded with to determine the percentage of light polarised, and it gave, as the mean of numerous determinations, the angle 36.6 deg, corresponding to a proportion of 16 per cent; 15 per cent, Professor Wright thinks, may be safely taken as the true value. The fact of polarisation implies that the light is reflected, either wholly or in part, and is thus derived originally from the sun. No bright lines were found in the spectrum, nor could any connection be traced between the zodiacal light and the polar aurova. This is important, as excluding from the possible causes of the light tho luminosity of gaseous matter, either.sponaneous or due to electrical discharge. Further, it cannot be supposed that the light is reflected from masses of gas or from globules of precipitated vapour, as ihe latter, in empty space, must eyapp'rate, and the former expand to too low a density to produce any effect on the rays. of Jiglrt, IJence Professor Wright concludes that the light is reflected from matter in the solid state from innumerable small bodies revolving about the sun in orbits, of which more lie in the neighborhood of the ecliptic than near.any other plane passing through the sun. These meteorites, which are in all probability similar in character to those which fall upon the earth, must bo either metallic bodies or stony masses. If we accept Zollner's conclusion, that the gases of the atmosphere must extend through the solar system, though in an extremely tenuous condition in space, the oxidation of metallic meteoroids would be merely a question of time. They would thus become capable of rendering polarised the light reflected from the plane, and the same effect would be produced by those of stony character. In order to ascertain whether the proportion of polarised light actually observed approached in any degree what might be expected from stony or. earthy masses, of a semi-crystalline character, with a granular structure and surfaces more or less rough, a large number of substances were submitted to examination with a polarirhetor; and the results showed that, from surfaces of this nature, the light reflected has in general but a low depth of polarisation j not greatly different in avrage from that of the zodiacal %ht. /v,',/;'.;-. : ;. ; ; Til- Usui's of the phenomenon, as discovered by Professor Wright, may therefore be summarised as f pllo\ys ; J| is light polarised in "a plane 'pacing .through the sun, to the nmpunj; of about 15 per cent. The spectrum is the same as, that of sunexcept in intensity. Its light is derived from the sun reflected on solid matter, which consists of small bodies re-, volving- about the sun in orbits crowded together towards tho ecliptic. I