ASTRONOMY, PROGRESS OP, IN 1891. 



51 



ere. At ICadras the halo could not be teen, though 



the central white spot, observed at several pre- 



ir.-ui.-iis was plainly seen l>y the late Prof. 



ii uiii I ,-ui assistant. 



mysterious [ihriidmenon, the "Black 

 l>ro|>," ii-nally seen at transits of Venus, also 



Hi-rally visible. It appears as a black liga- 

 ment i-i'iiMi-i-iinir at ingress and egress the limbs 

 of tin' planet and the sun, growing narrower and 

 narrower as the planet progresses; when the rupt- 

 n iv occurs, especially at ingress, the planet is 

 found to have advanced considerably, wnich ren- 

 ders it impossible to determine accurately the 

 prcci-f instant of contact. As to the cause of 

 the-e appearances astronomers are not agreed. 



Transits of Mercury will, for ages, occur in the 

 months of November and May, the next falling 

 due Nov. 10, 1894. 



Prof. Simon Newcomb, who has reviewed the 

 transit of Venus observations of the years 1761 

 and 1 ?<!), from which Encke deduced a parallax 

 of 8-571" and the earth's distance from the sun 

 to be about 95^ million miles, computes the par- 

 allax to be 8'79", which agrees almost exactly 

 with that obtained from the transits of 1874 and 



ml makes the sun's distance in round num- 

 bers 92,500,000 miles. 



Spectrum Analysis. Spectrum analysis, as 

 applied to astronomical investigation and the 

 interpretation of the solar, astral, and nebular 

 lines, is still the absorbing theme among astrono- 

 mers, and through the labors of such investiga- 

 tors as Huggins, Vogel, Rowland, Pickering, 

 Young, De la Rue, Draper, Janssen, Langley, 

 Kei'lcr, and scores of others equally well known, 

 it has become a distinct branch of science. Many 

 observatories have separate appliances for the 

 prosecution of this work, and the discovery of 

 the gelatin dry plate has lent invaluable aid by 

 making possible the photographing of their spec- 

 tra, and thus securing an imperishable record for 

 future comparison. 



As we have seen, the spectroscope has shown 

 that terrestrial matter is not confined to the earth, 

 but exists also in the sun and in every visible star 

 as well as in comets and the nebula?. 



The task of analyzing the stars and nebulae, 

 says Prof. Hujrgins. "is one of great difficulty 

 when we have to deal with spectra differing from 

 the solar type. We are thrown back upon the 

 laboratory for the information necessary to en- 

 able us to interpret the indications of the spec- 

 troscope as to the chemical nature, the density 

 and pressure, and the temperature of the celes- 

 tial spaces." 



The Nebular Line. The vexed question, 

 whether the principal nebular line is coincident 

 with the magnesium fluting, which has so agi- 

 tated the spectroscopists has been, it would 

 seem, settled by both Dr. Hnggina and Prof. 

 K'eeler. The latter fixes with great accuracy the 

 position of the former, and makes it 0-43 tenth 

 metre more refrangible than the lower ed ice of 

 the magnesium fluting. The nebular line, lie 

 declares, has no resemblance to a fluting. Flut- 

 iiiirs and lines of magnesium, which could not 

 fail to appear with the fluting at wave length 

 5006-36, are entirely absent in nebular spectra. 

 His conclusion that the principal nebular line is 

 not coincident with the magnesium fluting must 

 be regarded as conclusive. And this proof does 



not rest on observations of one nebula only, but 

 of several, and with a dinqwrsion often equivalent 

 to that of 24 prisms of 60, as the fourth spec- 

 trum of a Rowland grating of 14438 lines to the 

 inch was often employed. The whole matter 

 turns on an almost inappreciable difference of 

 the wave lengths of the two, that of the nebular 

 line being 5005-68 tenth metres and that of the 

 magnesium fluting 5006-13. The amount of dis- 

 placement of the lines being very small, it is 

 barely possible that it is due to the relative mo- 

 tion of the earth toward the apex of the sun's 

 way, or of the nebula) in the line of sight, or to 

 both causes. The conclusion arrived at by Dr. 

 and Mrs. Iluggins and Profs. Lieving, Dewar, 

 and Keeler effectually consigns to oblivion the 

 celebrated meteoric theory of Lockyer. 



Celestial Photography. The following 

 quotation from Prof. E. E. Barnard, of the Lick 

 Observatory, California, whose fine photographs 

 have attracted wide attention, will DC instruct- 

 ive to those who wish to photograph celestial 

 objects with a telescope : " The actinic image 

 is totally invisible on the ground glass, and we 

 have to grope for it, as it were. Its position can 

 easily be found by experiment. A suitable at- 

 tachment is made to carry the ground glass and 

 plate holder ; this takes the place of the eye- 

 piece, and is supposed to be adjusted for chang- 

 ing the focus. If the telescope is adjusted to a 

 star and allowed to remain stationary, the star 

 will pass across the field of view by the rotation 

 of the earth. Focus the image carefully on the 

 ground glass. It should appear as a tiny point 

 of light. Record this position of the tube. Sub- 

 stitute now the sensitive plate, and adjust the in- 

 strument so that the star shall cross the field ; 

 give an exposure of, say, half a minute, the tele- 

 scope remaining stationary. Draw the tube out 

 now about 0-05 of an inch, and repeat the ex- 

 posure; continue this for a number of times, 

 taking care after each exposure to shift the tele- 

 scope in altitude, so that successive trials shall 

 not fall on each other. When the plate is de- 

 veloped it will contain a series of lines or trails 

 produced by the light of the star as it crossed 

 the plate. Some of these will be blurred, but 

 it will be seen that they successively become 

 sharper until one is found that is perfectly sharp, 

 i. e., if the experiment has been carried far 

 enough. This will have been made at the chemi- 

 cal focus. The record for this trail, compared 

 with the reading when the image was in focus 

 on the ground glass, will be the correction to the 

 visual to obtain the chemical focus. Hence, 

 when a photograph is to be made, the image is 

 sharply focused on the ground glass, the tele- 

 scope is then adjusted to the chemical focus, and 

 the resulting picture should be sharp. I have 

 thus experimented with four different telescopes, 

 and found that they all gave verv satisfactory 

 photographs at the chemical focus.' 1 Prof. Bar- 

 nard found the chemical focus of each of the four 

 telescopes to be outside of the visual, 0'17. O'lO. 

 0-12, and 0-24 of an inch respectively. The la.-t 

 three were telescopes of the Lick Observatory. 



The task of photographing the planets is one 

 of great difficulty, inasmuch as the faint ness of 

 their light does not admit of instantaneous ex- 

 posures, as is the case in photographing the sun, 

 moon, and the brightest stars. Besides, the 



