ASTRONOMICAL PHENOMENA AND PROGRESS. 



43 



measurement. Prof. Peirce aptly says : " This addi- 

 tion to astronomical research is unsurpassed by any 

 step of the kind that has ever been taken. The 

 photographs afford just as good an opportunity for 

 new and original investigation of the relative posi- 

 tion of near stars as could be derived from the stars 

 themselves as seen through the most powerful tele- 

 scopes. They are indisputable facts, unbiassed by 

 personal defects of observation, and which convey 

 to all future times the actual places of the stars when 

 the photographs were taken." 



Mr. Asaph Hall, who shared with Prof. Bond the 

 work of measuring the photographic images, and of 

 reducing the measurements, has very recently sub- 

 jected the photographic method to a critical com- 

 parison, with a view to deciding on its_ value when 

 applied to the observation of the transit of Venus. 

 He appears, as regards its application to stellar ob- 

 servations, to under-estimate the photographic meth- 

 od in consequence of want of rapidity ; but he ad- 

 mits that in the case of a solar eclipse, or of the 

 transit of a planet over the sun's disk, it has very 

 great advantages, especially over eye-observations of 

 contacts, inner and outer, of the planet and the sun's 

 limb, and that the errors to which it is subject are 

 worthy of the most thorough investigation; The 

 observation of a contact is uncertain on account of 

 irradiation, and is also only momentary : so that, if 

 missed from any cause, the record of the event is 

 irretrievably lost at a particular station, and long 

 and costly preparations rendered futile. On the 

 other hand, when the sky is clear, a photographic 

 image can be obtained in an instant, and repeated 

 throughout the progress of the transit, and, even if 

 all the contacts be lost, equally valuable results will be 

 secured, if the data collected on the photographic 

 plates can be correctly reduced, as will be proved 

 hereafter to be undoubtedly possible. That the 

 transit of Venus will be recorded by photography 



ords. There is also a possibility of Portugal taking 

 part in these observations ; for it is contemplated by 

 Senhor Capello to transport the Lisbon photohelio- 

 graph to Macao. There are at present five photo- 

 heliographs in process of construction for the ob- 

 serving parties to be sent out by the British Govern- 

 ment, under the direction of the Astronomer Koyal, 

 Sir George B. Airy. The Eussian Government will 

 supply their own parties with three similar instru- 

 ments ; and I am also having constructed one of my 

 own for this purpose and for future solar observa- 

 tions. All these instruments, made precisely alike, 

 will embody the results of our experience gained 

 during the last ten years in photoheliography at the 

 Kew Observatory while belonging to this Associa- 

 tion. One only of them, namely, the photohelio- 

 graph which has been at work for some years at 

 Wilna, is of somewhat older pattern ; but how great 

 an advance even this instrument is on the original at 

 Kew is proved by the delightful definition of the 

 most delicate markings of the sun in the pictures 

 which have reached this country from Wilna. 



To give some idea of the relative apparent magni- 

 tudes of the sun and Venus, I may mention that at 

 the epoch of the transit of 1874 the solar disk^would, 

 in the Kew photoheliograph, have a semi-diameter 

 of 1965.8 thousandths of an inch, or nearly two 

 inches ; Venus a semi-diameter of 63.33 of these 

 units ; and the parallax of Venus referred to the sun 

 would be represented by 47.85 such units, the maxi- 

 mum possible displacement being 95.7 units, or near- 

 ly one-tenth of an inch. 



The Zodiacal Light. Prof. C. Piazzi- Smyth, 

 Astronomer Royal for Scotland, improved the 

 occasion of a trip to Palermo to study the 

 zodiacal light, which is seen with remarkable 

 distinctness in that part of the world. The 



time of the observations was March, and he 

 had at his disposal the best instruments be- 

 longing to the Spectroscopic Society of Italy. 

 In a paper, read before the Royal Astronomi- 

 cal Society, in June, after describing the care- 

 ful arrangements made to secure the most 

 favorable view, he continues : 



There was the zodiacal light brilliant, so to speak, 

 instantly recognized by all of us as being, as it 

 should be for the vernal equinox, in latitude 38 

 north, and vastly brighter than it ever could be seen 

 in any very northern city, as of 50 or 60 latitude. 

 Upon that most visible zodiacal light, then, we di- 

 rected the grand tube of the spectroscope, its slit 

 being arranged narrow, or as most suitable to resolv- 

 ing the reference of blue flame-bands into lines ; or, 

 as I may add, from long Edinburgh experience, for 

 seeing and identifying the one bright line formed by 

 the peculiarly monochromatic light of ordinary green 

 aurora. On looking into the eye-piece, there were 

 all these bands of the reference spectrum, with 

 symptoms of linear resolution, together with the 

 lithium and sodium lines thin, as representing the 

 fine slit, ranged across the lower part of the field ; 

 but, in the upper part of the field, where the spec- 

 trum of the zodiacal light ought to have appeared, 

 there was nothing. To make quite sure, the instru- 

 ment, carrying the reference-lamp along with it, was 

 moved slowly, first in azimuth, and then in altitude, 

 backward and forward, across all the brightest parts 

 of the zodiacal light ; the reference-lamp, too, was 

 dulled, and at last altogether excluded, but still noth- 

 ing whatever was seen, on looking, with the prisms, 

 through the slit, at the zodiacal light, which other- 

 wise was not only abundantly bright to the naked 

 eye, but also when viewed through the same prisms, 

 if used without any slit at all. 



The only explanation, then, that is possible for 

 the non-appearance of any spectral light, whether in 

 lines, bands, or any thing else, when the fine slit 

 was used, is, that the light of the zodiacal manifesta- 

 tion is not, as has been sometimes asserted, mono- . 

 chromatic light of one definite refrangibility (in 

 which case it could only give a line as bright as the 

 full phenomenon, and as narrow as the slit), but it is 

 of many various refrangibilities, and spread thereby 

 over so large a spectral range as to become weakened 

 down to practical invisibility. 



To ascertain over how great a spectral range the 

 zodiacal light is spread, the slit was opened slowly 

 until something was seen in the dark field, and then 

 it was no line or lines, but a short portion of contin- 

 uous spectrum that was indicated, becoming clearer, 

 and rather larger too, when the slit was still further 

 widened. Indeed, we made it at last even extrava- 

 gantly wide, or approximating in breadth to the 

 length of the said portion of continuous spectrum 

 itself. 



This portion, however, was never definite, never 

 bounded by sharp, upright, parallel lines, as were 

 the lines of lithium and sodium, with that broad 

 slit ; but it was shaded off gradually into darkness 

 at each end, and had its general maximum of light 

 nearer the less refrangible end. Moreover, it was 

 altogether so faint, as well as undefined and indefin- 

 able at the same time, that it could never be seen 

 certainly in conjunction with the reference-spectrum, 

 the latter having to be excluded, and then let in 

 again, so that the comparison of the two might be 

 made by memory assisting the eye. 



Hence he infers that the older astronomical theory 

 of the zodiacal light being the solar illumination of 

 infinitely small, distant particles, such as meteors re- 

 volving about the sun, whether in orbits of infra or 

 ultra planetary ellipticity, is spectroscopically main- 

 tained, while that such solar zodiacal light has any 

 physical connection with the essentially terrestrial 

 accompaniment of aurora is just as eminently nega- 



