68 



NATURE 



\_N0V. 2 2, 1877 



rings, being really much smaller than the apparent size 

 due to physiological effects, and of this latter size every 

 star must appear. 



The accompanying woodcuts of a region in the con- 

 stellation of Gemini as seen with the naked eye and with 

 a powerful telescope will give a better idea than mere 

 language can do of the effect of this so-called space- 

 penetrating power. 



With nebulae and comets matters are different, for 

 these, even with small telescopes and low powers, often 

 occupy an appreciable space on the retina. On increasing 

 the aperture we must also increase the power of the eye- 

 piece, in order that the more divergent cones of light 

 from each point of the image shall enter the pupil, and 

 therefore increase the area on the retina, over which the 

 increased amount of light, due to greater aperture, is 

 spread ; the brightness, therefore, is not increased, unless 

 indeed we were at the first using an unnecessary high 

 power. On the other hand, if we lengthen the focus of 

 the object-glass and increase its aperture the divergence 

 of the cones of light is not increased and the eye-piece 

 need not be altered, but the image at the focus of the 

 object-glass is increased in size by the increase of focal 

 length, and the image on the retina also increases as in 

 the last case. We may therefore conclude that no comet 

 or nebula of appreciable diameter, as seen through a tele- 

 scope having an eye-piece of just such a focal length as 

 to admit all the rays to the eye, can be made brighter by 

 any increase of power, although it may easily be made to 

 appear larger. 



Very beautiful drawings of the nebula of Orfon and of 

 other nebulae, as seen by Lord Rosse in his 6-foot 

 reflector, and by the American astronomers with their 

 26-inch refractor, have been given to the world. 



The magnificent nebula of Orion is scarcely visible to 

 the naked eye ; one can just see it glimmering on a fine 

 night ; but when a powerful telescope is used it is by far 

 the most glorious object of its class in the northern hemi- 

 sphere, and surpassed only by that surrounding the 

 variable star r] Argus in the southern. And although, of 

 course, the beauty and vastness of this stupendous and 

 remote object increase with the increased power of the 

 instrument brought to bear upon it, a large aperture is 

 not needed to render it a most impressive and awe- 

 inspiring object to the beholder. In an ordinary 5-foot 

 achromatic many of its details are to be seen under 

 favourable atmospheric conditions. 



Those who are desirous of studying its appearance, as 

 seen in the most powerful telescopes, are referred to the 

 plate in Sir John Herichel's " Results of Astronomical 

 Observations at the Cape of Good Hope," in which all its 

 features are admirably delineated, and the positions of 

 150 stars which surround 6 in the area occupied by the 

 nebula laid down. In Fig, 4 it is represented in great 

 detail, as seen with the included small stars, all of which 

 have been mapped with reference to their positions and 

 brightness. This, then, comes from that power of the 

 telescope which simply makes it a sort of large eye. We 

 may measure the illuminating power of the telescope by a 

 reference to the size of our own eye. If one takes the 

 pupil of an ordinary eye to be something like the fifth of 

 an inch in diameter, which in some cases is an extreme 

 estimate we shall find that its area would be roughly about 

 one-thirtieth part of an inch. If we take Lord Rosse's 

 speculum of six feet in diameter the area will be some- 

 thing like 4,000 inches ; and if we multiply the two to- 

 gether we shall find, if we lose no light, we should get 

 120,000 times more light from Lord Rosse's telescope 

 than we do from our unaided, eye, everything supposed 

 perfect. 



Let us consider for a moment what this means ; let us 

 take a case in point. Suppose that owing to imperfec- 

 tions in reflection and other matters two-thirds of the hght 

 is lost so that the eye receives 40,000 times the amount 



given by the unaided vision, then a sixth magnitude star — 

 a star just visible to the naked eye — would have 40,000 

 times more light, and it might be removed to a distance 

 200 times as great as it at present is and still be visible 

 in the field of the telescope just as it at present is to the 

 unaided eye. Can we judge how far off the stars are that 

 are only just visible with Lord Rosse's instrument ? Light 

 travels at the rate of 185,000 miles a second, and from the 

 nearest star it takes some 3^ years for light to reach us, 

 and we shall be within bounds when we say that it will 

 take light 300 years to reach us from many a sixth magni- 

 tude star. 



But we may remove this star 200 times further away and 

 yet see it with the telescope, so that we can probably see 

 stars so far off that light takes 60,000 years to reach us, 

 and when we gaze at the heavens at night we are viewing 

 the stars not as they are at that moment, but as they were 

 years or even hundreds of years ago, and when we call to 

 our assistance the telescope the years become thousands 

 and tens of thousands — expressed in miles these distances 

 become too great for the imagination to grasp; yet we 

 actually look into this vast abyss of space and see the 

 laws of gravitation holding good there, and calculate the 

 orbit of one star about another. 



J, Norman Lockyer 



(To be continued. ) 



ZOOLOGICAL GARDENS^ 

 'T^HE lists and reports of the various zoological gardens 

 -■■ now before us show that much progress has lately 

 been made by these as by other institutions connected 

 with natural history. For though zoological gardens are 

 looked upon by many as a simple form of amusement 

 there can be no question that, when rightly conducted, 

 they are not only mstructive in the highest degree, but 

 also tend materially to advance the interests of the higher 

 branches of natural science. All persons, therefore, who 

 take an interest in the progress of science will be glad to 

 see the number of zoological gardens increasing among 

 the dependencies of this country and in other States. 



Of the first of the five works on our list we need say 

 but little. The Gardens of the Zoological Society of 

 London, in the Regent's Park, are too well known to 

 most of our readers to require a lengthened notice. The 

 chief additions to their unrivalled menagerie are recorded 

 every week in our columns. The volume now before us 

 contains a catalogue of all the species of vertebrated 

 animals, of which examples have been exhibited during 

 the past fifteen years, arranged in systematic order. The 

 various specimens are distinguished by letters, and the 

 date and mode of acquisition of each individual are added. 

 Thirty-five woodcuts, most of which have originally 

 appeared in the Society's Proceedim^s, illustrate some of 

 the more remarkable forms. The result shows that from 

 the commencement of the year 1861 to the close of 1875, 

 there have been obtained for the collection in the Regent's 

 Park, examples of no less than 2,143 species of vertebrated 

 animals. Of these 570 were mammals, 1,224 birds, 227 

 reptiles, 39 batrachians, and 83 fishes. 



The catalogue of the animals in the newly-established 

 Zoological Gardens at Calcutta, concerning the foundation 

 and progress of which we have written at full length not 

 long since,^ is next upon our list. It is drawn up after 



^ (i) List of Vertebrated Animals now or lately living in the Gardens of 

 the Zoological Society of London. Sixth Edition. 1877. (London : 

 Longmans). 



(2) Li.st of Vertebrated Animals living in the Zoological Gardens, Calcutta, 

 April, 1877. Printed at the Bengal Secretarial Presii. 1877. 8vo. 



(3) A Guide to the People s Park, Madras, wiih a description of the 

 Zoological Collection contained therein. (Madras : Higgiubotham and Co , 

 1876) 



(4) The Fifth Annual Report of the Board of Directors of the Zoological 

 Society of Philadelphia. Read at the Annual Meeting cf the Members aud 

 Loanholders ol the Society, April 26, 1877. 8vo. (Philadelphia, 1877 ) 



(5) Report of the Director ot the Central Fark Menag-rie, Department of 

 Public Pa-ks, City of New York, fer year i8;6. iNcw Yoik, 1877 : B. M. 

 Lees. Printer, 210, Fulton St-eet. ) 



^ Nature, vol xvi. p. 28. 



