June 1, 1891. 



KNOWLEDGE 



115 



incapacity is certainly an acquired one in the last-named 

 bird, and there is a considerable probability that it was 

 liicewise so in the two former. 



The great peculiarity whereby Birds differ from all 

 other animals is iu the presence of their external covering 

 of feathei-s. A feather, as we all know, is one of the most 

 iieautiful objects in nature ; and its structure, which we 

 may, perhaps, explain in a later article, is an admirable 

 instance of adaptation for a particular purpose. The uses 

 of feathers are two-fold. In the first place; the small 

 ones with which the body is clothed form the most 

 perfect covering that can be imagined to ensure the main- 

 tenance of the high bodily temperature so essential to the 

 active existence of a bird. Then, again, the larger and 

 stronger feathers of the wings are the most efficient 

 instruments for obtaining the utmost advantage from the 

 resistance of the air to their strokes during flight. The 

 peculiar nature of tlie wings of Birds may be summarised 

 by the statement that whereas all other animals Hy by 

 means of expansions of the skin itself, these alone tly 

 iiy means of sejiarate outgrowths or processes developed 

 from the skin. 



(To be continued. ) 



ilcttcvs. 



[The Editor does not hold himself responsible for the opinions 

 statements of correspondents.] 



photographer must be naturally an experimenter, his 

 difficulties must be thought out and conquered as they 

 arise. He will soon find the exposures necessary to 

 obtain the best ell'ects. For the Jloon, .Tupiter and Saturn, 

 in the principal focus, only a fraction of a second is 

 necessary, depending on the aperture and focal length 

 of the telescope, as well as on the altitude of the object 

 above the horizon and the clearness of the uiglit. Hence, 

 these objects, as well as the Sun, can be photographed 

 without a driving clock. Dr. von Konkoly'sl book on 

 Astronomical Photography contains a good many woodcuts 

 of instruments and apparatus, but they are not such 

 diagrams that a reader who did not know what was 

 represented could construct an instrument from. This 

 book is also four years old, and Astronomical Photo- 

 graphy has made great progress since that time. I would 

 advise any intending astronomical photographer to 

 thoroughly read some book on geometrical optics, and 

 then to think out his difficulties for himself. — A. C. 

 Ran YARD.] 



To the Editor of Knowledge. 

 Mackay, Queensland, March 21st, 1801. 



Sir, — Is there any Kmilisli work treating of Astronomical 

 Photography '? 



In ti. L. C'hambers' " Handbook of Astronomy," Vol. II., 

 1800, p. 116, a footnote mentions a German work, 

 Konkoly's, published by Halle, 1887, as the only work on 

 the subject. 



People who live iu the centres of population can, no 

 doubt, get vied riM-e instruction ; those in distant parts of 

 the world have to depend upon books, and there seem to 

 be none on this subject. 



I am one of those who deeply regretted the cessation of 

 Answers to Correspondents in 1885, and of Gossip in 1888, 

 which alone were worth, to me, twice the money paid for 

 Knowledge. I liked the genially caustic pen of the late 

 lamented R. A. Proctor, and his way of making personal 

 friends of his readers. Your obedient servant, 



J. GwEN Davidson. 



[We have not at present, as far as I am aware, any book 

 in Isnglish on Astronomical Photography. A very charm- 

 ingly illustrated little French book on the subject was 

 issued in 1.SK7 by Admiral Mouchez,- the Director of 

 the Paris Observatory, but it is devoted to giving an 

 account of what has been done, rather than to answering 

 the questions which I imagine Mr. Davidson would like 

 to ask. The photographic difficulties which the astronomical 

 photogi'apher will encounter arc dealt with in a legion 

 of text-books. The astronomical dillicullies involved 

 in the accurate mounting of his equatorial, and the 

 accurate driving of his clock, cannot, I think, be dealt with 

 hy any copy-book rules. To succeed, the astronomical 



* " liii rhiitdfiraiiliic Aslroiiciiii 

 M. Ic ('.Mitr.'.Aiiiinil K. Mciu-luv 

 dew Umiuls-.\ujriistiii, JSST. 



(|110 !l rOl.S.'VVll 



INu-is, (iiiulli 



.lircdr Paris." I'nr 

 rn\i liars, .'55, tjimi 



To the Editor of Knowledge. 



Dear Sir, — Whilst grateful to you and your reviewer 

 for pointing out the mistake in the last edition of my 

 Celextial Mutiona (1 might, perhaps, demur to some of the 

 remarks, but do not propose to do so at present), allow me 

 to call your attention to an error in the same number of 

 Knowledge itself (p. 01), which may puzzle many readers. 

 The passage runs : " He [Michell in I'hil. Tntnx. for 1767] 

 concludes that there must be some physical connection 

 between the numerous double and triple stars which had 

 already l)een discovered by Sir WilUam Herschel . . . ." 

 Michell makes no mention in his paper of HerschePs 

 discoveries, nor could he for a similar reason to that which 

 prevented a distinguished personage from seeing a Spanish 

 fleet. It was not in sight; and HerschePs discoveries had 

 not commenced in 1767, the year after he was appointed 

 organist in Batli. 



Yours faithfullv, 

 P.lackheatli, May (ith, iSOl. W. T. Lynn. 



[Michell's reference to the double and triple stars dis- 

 covered by Herschel, is in ^ paper published in the I'hit. 

 Trans, for 1781. The fact to which I wish to call atten- 

 tion is that Michell's remarkable papers were written 

 before Herschel's discovery that several close pairs of stars 

 were moving round one another. The boldness of Michell's 

 conclusion that there must be a physical connection be- 

 tween close double stars is rendered more remarkable by 

 the fact that, at the date of his first paper, 1767, less than 

 a hundred of such pairs of stars were known. Michell's 

 words in his first paper are well worthy of being quoted at 

 length. Ho says, at p. 217 of the Vint. Trans, for 1767, 

 after speaking of certain stars which appear double to the 

 naked eye — 



" If, besides these examples that are obvious to the 

 naked eye, we extend the same argumeui to the smaller 

 stars .... which appear double, treble, &c., when seen 

 through telescopes, we shall find it still infinitely more 

 conclusive, both in the particular instances and in the 

 general analogy arising from the frequency of them. We 

 may from hence, therefore, with the highest probability, 

 conclude (the odds against the contrary opinion being 



t " PTOktisrlio .Viileitung zur Iliminelsphotognvpliio ncbst oilier 

 kurgefassteii Aiiloitiiii!; zur nuHlornpii pboUigniptiiselu'ii Oponitiou 

 und dor spoctrul pluilograpliic ini oabiiiot von Niooluus von Koiikoiy. 

 Uallo, 1SH7. 



