140 



♦ KNOWLEDGE ♦ 



[March 1, 1886. 



rare in the Old World. Little trace of the Cretaceous 

 land-areas remain, but the plants of the upper strata 

 resemble existing yegetation ; in the words of the science 

 books, Angiospeimious exogens appear, which in the 

 vulgar tongue means leaf-bearing trees having a true 

 bark (except cone-bearers), and growing from the outside, 

 ■with their seeds enclosed in a vessel, as the oak, beech, 

 ■willow, fig, walnut. They are called " exogens " in 

 contrast to "endogens," or palms, grasses, and lilies, 

 which have no true bark, and grow by additions from 

 the inside. 



Of the total thickne.ss of the stratified rocks, estimated 

 ,it 1. '50,000 feet, the Secondary .sy.stems occupy only 

 l.'i,00O, or Sjrds of the whole. But, as this abstract of 

 their contents shows, their importance is not to be 

 measured by the space which they fill. Whether or not 

 trinsitional forms, which from their nature h.ad a shorter 

 range of time and less chance of preservation, existed 

 in the Primary systems, we cannot say, for life is older 

 than its records. But it is in the Secondary, when, as 

 the coal-seams and coral deposits of extreme northern 

 zones show, warm climates prevailed, that the marked 

 advance in specialisation of plant and animal forms is 

 manifest. 



PHOTOGRAPHING FIFTEEN MILLION STARS, 

 Br Richard A. Proctor. 



'■■' "■" MAGNIFICENT suggestion has been 



made by French astronomers. Already 

 in these columns the work done by the 

 jthotographic eyes of science, directed 

 towards the heavenly bodies, has been 

 dealt with. By the power of instan- 



taneous vision which the photogra]ihic 



eye, unlike the human eye, possesses, the sun's cloud- 

 laden surface has been delineated, despite the constant 

 tluetuations of the air- through which the sun has to 

 be viewed. By the power of selecting special colours 

 wherewith to work, the photographic eye has dra'^n 

 the corona when no trace of that .solar appendage has 

 been visible to ordinary eyesight. The delicate features 

 of the star-clouds have been depicted, through the power 

 which the photographic eye possesses, of seeing more 

 and ^ more by long - continued gazing upon faintly 

 luminous objects. And now it is j)roposed to do what 

 assui-edly no astronomer, nor any band of astronomers, 

 could hope to clfect, even if working for the whole 

 dur.ition of the longest; life. It is propo.sed to chart in 

 their true positions all the twenty millions or so of 

 stars which are included in the first fifteen magnitudes, 

 so that the astronomers of future generations may know 

 for certain the aspect of the stellar heavens— to that 

 vast depth, at least— towards the close of the nineteenth 

 century. Let us see what are the conditions cf the 

 task. 



Using a telescope provided -svith a specially jirepared 

 object-glass of about 1.3 inches in diameter, MM. Paul 

 and Prosper Henry have been able to take in a single 

 hour photographic charts of spaces in the heavens ex- 

 tending 3 degrees in length and 2| degrees in breadth— 

 siy six moon-breadths by four and a half. Their actual 

 plan h.as been to give in each case three exposures, with 

 such slight displacements that each star is tripled, and so 

 therj can be no possibility of mistaking accidental dots 

 on the plate for stars ia the heavens. (It might be well, 

 however, if in the photographs iinally prepared only 

 single images of each star were given, a preparatory 



plate with triple images serving for the correction of the 

 one finally prepared, which might have three hours of 

 exposur.' without displacement.) Now, a space of 

 3 degrees by l'| degrees on the heavens, cr G| square 

 degrees, is about 1-6, 112th part of the whole star-sphere. 

 So that if twelve observatories, in different parts of the 

 northern and southern hemisphere, were employed to 

 photograph the star-sphere on one and the same plan, 

 then at each observatory only about .510 plates would 

 have to be made. Counting about fifty-one moonless 

 nights of clear sky and still air, one night being given to 

 e:ich plate, the whole work would be completed in ten 

 years. If the charts thus obtained could be combined in 

 sets of four, in the manner already employed by MM. 

 Henry, there would be 1,^)2^ sheets, each representing a 

 portion of the heavens, extending 6 degrees by 4J ; but 

 although Admiral Moucliez suggests this plan as de- 

 sirable, it appears open to excejition on account of change 

 of scf.le near the edges of the plates. 



The number of star.s which would probablj* be shown 

 in this splendid contribution to the astronomy of the 

 future would be about twenty millions. In a single 

 plate, obtained by MJI. Henry recently, nearly 5,000 star.s 

 can be counted ; and if 0,112 gave each such a number — ■ 

 say 6,000 times .'3, 000 — that would be 30 millions of stars. 

 But the region shown in this particular plate belongs to 

 a rich part of the Milky Way, c.,nd it has been shown 

 by my chart of 324,000 northern stars down to the 

 10th magnitude, that there is a much greater density 

 of stellar aggregation on the jSlilky Way long before the 

 space-penetrating powers have been employed which the 

 Herschels thought probably necessary to reach the regions 

 whence the nebulous light of the Milky Way was sup- 

 posed to proceed. If the jihotographic method were 

 applied uniformly over the whole heavens, with a space- 

 penotrating jiower reaching stars of the l.')th magnitude 

 in all directions, it is jirobable that about 20.000,000 

 stars would be shown. The great gauging telescopes 

 used by the Herschels would show at the very least 

 100,000,000- or rather would liave shown that number 

 if it had been possible to bring every portion of the star- 

 sphere under their survej'. 



A new err, of stellar astronomy will open ■v\ith this 

 photographic work. The problems connected with the 

 architecture of the heavens, hitherto dealt with by 

 very imperfect methods, will now be discussed with all 

 the advant,"ge of at least a perfect system of survey. 

 It is impossilile, indeed, to overestimate the advantage of 

 a system of charting over all the methods of statistical 

 research which astronomers furnierly emjiloyed. William 

 Herschel in his first method counted all the stars which 

 one and the same telescope — a very powerful one, 18 in. 

 in diameter — would show in different directions. He 

 could only take a field of view here and a field tf view 

 there, not many hundreds iu all, his son and worth j- suc- 

 cessor in the work making similar observations in the 

 southern hemisjihere. No peculiarities cif arrangement, 

 nothing, in fact, but the roughest features of stellar 

 distribution, could be recognised by such a method as 

 thi.s. It showed, however, how vast the number of stars 

 formiag our galaxy is, and it satisfied Sir W. Herschel 

 that the assumption by which he had proposed to 

 interpret his numerical gauges was inadmissible, the 

 stars not being strewn throughout our galaxy with 

 any approach to uuiformit}'. Herschels second method, 

 commonly confounded with his first (insomuch that 

 one may often find even men like Ai-ago and Humboldt 

 mixing up in the same paragraphs the results of 

 both methods of observation) was entirely different. He 



