202 



KNOWLEDGE 



[Septkmbeb, 1902. 



means of observing the star with the naked eye, but to 

 enable Ibe oliscrver to measure as accurately as possible 

 the distance of the star from the true zenith at the moment 

 of trausit. 



Sir John Herschel mentions a case, which he considers 

 as satisfactory evidence, of an onticiaa who stated that 

 the earliest circumstance that drew his attention to 

 astronomy " was the regular appearance at a certain hour 

 for several successive days, of a considerable star through 

 the shaft of a chimney."" This, it will be noticed, is second- 

 hand evidence. I have never been able to obtain evidence 

 even so direct as this myself, though I have met several 

 persons who felt quite confident that tliey had seen stars 

 by daylight on looking up the shaft of a mine, or that 

 " some one bad told them he had done so." 



But the value of such indefinite statements is nil. I 

 have met evidence more direct and explicit in support of 

 that favourite legend due to the fertile imagination of the 

 Emperor Jahangir, of the Indian juggler who threw a 

 rope into the air and climbed up it, to disappear at its top 

 into space — a legend which still makes periodic reappear- 

 ances, and finds not a few devout believers. But direct, 

 first-hand, scientific testimony of an observer who has been 

 enabled by the use of a shaft to detect stars with the 

 naked eyeat midday is still to seek. By scientific testi- 

 mony, I mean the record of the day, hour and minute 

 when the star was seen, the latitude of the place, the 

 depth of the shaft, the breadth of its mouth — the numerical 

 elements, in a word, which are necessary to give value to 

 the observations. There must be not a few of the many 

 who take an intei-est in astronomy to whom the means for 

 making such an observation are available, and who, if 

 they would take the trouble, could report, " I have seen 

 such a star at such a time," or, "I have watched for such 

 a star at the time of its transit across the z.enith on so 

 many occasions, when the sky was clear, and could see 

 nothing." Such observations would set the question at 

 rest whichever way they tended ; but what are wanted, 

 here as elsewhere, are definite observations, carefully made, 

 fully and systematically recorded, not vague, second-hand 

 impressions which are perfectly valueless as evidence. 



Whether or no the use of a shaft to diminish the effect 

 of sky illumination, and so to render the stars visible by 

 daylight is ])racticable, it suggests a method for dealing 

 with what Prof. Newcomb in a i-ecent paper has justly 

 described " as among the most important fundamental 

 constants of astrophysics," namely, the value of the total 

 light of all the stars. In the paper* alluded to. Prof. 

 Newcomb points out that the " total amount of light 

 received from all the stars may serve as a control on 

 theories on the structure of the universe, because the 

 amount of light resulting from any theory should agree 

 with the observed amount. It is also a quantity which 

 we must regard as remaining constant from age to age." 

 Tet, strangely enough, few if any attempts have been made 

 to determine it. One of these was made by Mr. G-avin J. 

 Burns,t his method being to compare the brightness of 

 the sjiurious disc of a star seen out of focus iu a telescope 

 with the light of the sky. The eyepiece of the telescope 

 was pushed iu and out until the brightness of the spurious 

 disc seemed to correspond with that of the sky. Prof. 

 Newcomb, two years later, adopted several plans, his 

 purpose being a twofold one — first to determine tlie relative 

 brightness of different portions of the sky, and next to 

 express the brightnesses of given units of surfaces in terms 

 of starlight, from whence iu turn the brightness of the 

 whole heavens in terms of starlight may be inferred. 



* Astropht/sical Journal, December, 1901. 

 t Journal Brit. Ass. Asc, Vol. XII., p. 212. 



Prof. Newconib's first plan was to use a small tube, the 

 length of which he could easily vary, the ends of the tube 

 being covered with caps having apertures of varying 

 diameters, and to measure therewith the smallest area of 

 sky which was certainly visible to him. A second method 

 was by means of small mirrors arranged so as to enable 

 different regions of the sky to be compared directly. 

 Roughly speaking, the Galaxy appeared, surface for 

 surface, about twi(;e as bright as the sky outside it. 



For the determination of the brightness of different 

 areas in term.s of starlight a concave lens was used, so as 

 to spread out the image of the star into a disc, and the 

 brightness of the expanded image was cut down by means 

 of an absorbing glass to that of the sky. The results 

 appeared to point to a value for the total starlight of 

 over GOO to 800 stars of magnitude 0, whilst Mr. Gavin 

 Burns, by different methods, fixed the value at about 400 

 stars for one hemisphere, or 800 for the entire heavens. 

 Both results, though more accordant than might have been 

 expected, can only be regarded as first approximations, 

 and there is abundant room for many other observers to 

 follow these pioneers, and supplement their work. 



One method for comparing the light of the sky in two 

 different regions would be by means of some such simple 

 apparatus as the following: — A tube, bent at right angles, 

 should be fitted at the angle with a piece of card placed 

 at 4-!)° to either arm. The card should be painted a dead 

 black all except a white cross in the centre. The observer 

 should look down one arm, through a diaphragm about 

 i inch in diameter, and view the card, which would be 

 illuminated by the light coming down the other arm, and 

 the opening of which would be directed to some known 

 region in the sky. This opening should be provided with 

 a series of caps having apertures of different diameter, 

 and the arm itself should be fitted with a draw tube, so 

 that both the size of the opening and its distance from the 

 card might be varied at will. The observer, having care- 

 fully set the tube in some given direction, would move the 

 draw tube in or out, or vary the caps over the aperture, 

 until the white cross on the cardboard in the angle 

 could just be certainly discerned. The aperture, the 

 length of the draw tube, and the part of the sky to which 

 it is directed, must then be carefully recorded. 



The rough altazimuth, described in the chapter on 

 " Observations of the Sun,"* would prove a suitable 

 mount for such an instrument. If used for this purpose 

 its circles must be read, whilst, of course, the time of the 

 observation should be taken, and the state of the sky 

 noted. Necessarily, observations of this kind are only 

 possible at stations far from the glare of towns, and on 

 moonless nights of special clearness. 



The observer might well begin his work with some such 

 device as this, but in a field so nearly new there would be 

 full scope for his best ingenuity and contrivance in im- 

 proving on this beginning, and iu arranging for better 

 and exacter methods for dealing with the noble jiroblem 

 he had undertaken. 



It must be noted that the result of these observations 

 will give the sum of starlight -1- any other general source 

 of illumination which may be present. It must be assumed 

 that the observer is working far from the influence of any 

 artificial lights, and that so far as he can ascertain there is 

 absolutely no cloud or mist iu the sky. But there still 

 remains the question whether the general illumination of 

 the sky does not vary from time to time. Thus, two 

 observers of the very first rank, Mr. Denning and Mr. 

 Backhouse, have recorded that in August, lS80, the sky 

 was unusually light. It is clear that the sum total of 



* Kkowledge, 1900, June. 



