Mat 1, 1894.] 



KNOWLEDGE 



97 



^^^ AN ILLUSTRATED "^^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: MAY 1, 1894. 



CONTENTS. 



Magnifying Powers most useful in observing with 

 Large Telescopes. Bv Piol'. E. E. Barnard, of the 

 Lick Obsers-atory. and Mr. S. W. Bvbnham 



On the Mounting of Large Reflecting Telescopes. 

 Bv Sir HowAKD Geubb, F.R.S., &c., &l'. ... 



Streams of Stars in the Milky Way. By A. C. Eaxtaed 



Science Notes 



Insect Secretions.— I. E_t E. A. Butlke, B.A., B.Sc. ... 



Ancient and Modern Hippopotami. By R. Ltdekker, 

 B.A.Cantab 



Nal<ed Eye Double Stars discovered by Burnham ... 



The Luminiferous Ether. By J. J. Stewart, B.A.Cantab., 

 B Sc.Lond. 



What is a Comets Tall ? By A. C. RAirrABD 



Letters: — J. P. MaCLEAR; A. E. WnlTEHOrSE ; L. KiTDAUX; 



A. P. Skene ; VAr&HAjr Cornish ; T. W. Backhoxtse ; 

 J. S. ; Edw. G. Gilbert, M.D 



The Face of the Sky for May. By Herbert Sadler, 

 F.E.A.S. 



Chess Column. By C. D. Locock, B.A.Oiou 



9S 

 101 

 102 

 103 



106 

 111) 



111 

 113 



115 



118 

 119 



MAGNIFYING POWERS MOST USEFUL IN 

 OBSERVING WITH LARGE TELESCOPES. 



By Prof. E. E. Barnakd, of the Lick Observatory. 



M' 



■ R. EANYAED is quite right about the superiority 

 of small powers for planetary work with a large 

 telescope. I have always used the full aperture 

 of the thirty-six-inch in my work. It is possible, 

 however, that planetary detail might be somewhat 

 improved with a slight reduction of aperture. 



My experience with the higher powers has been 

 principally in the measurement of a list of close and 

 difficult double stars which I am observing at the request 

 of Mr. Burnham. What experience I have had with the 

 thirty -six-inch has been very varied. It has consisted in 

 the observation of the dovible stars referred to, the 

 measurement of the positions of very faint and distant 

 comets, observations of nebulae, of planetary details, and 

 the positions of satelUtes. 



On double stars I have found it always satisfactory to use 

 one thousand diameters if the night will stand it, because 

 of the greater scale and ease of measurement. I have used 

 two thousand six hundred with perfect satisfaction, but 

 this requires a perfect night and is only useful where the 

 stars are bright and not too unequal. With very close 

 stars I should always prefer this power when the night will 

 permit its use. 



Mr. Burnham's great experience in this line would, 



however, make Viim far more competent to speak of the 

 powers useful for this class of work with the great 

 telescope than I am. 



In reference to the planets, I can never get any satis- 

 faction from the surface markings of .Jupiter with as high 

 a power as one thousand, not even on the best nights. 

 Only the coarser details can then be made out. I find it 

 seldom satisfactory to use even five hundred and twenty. 

 The lower power of three hundred and fifty is always 

 preferable on this planet, and a similar remark also applies 

 to Mars. Five hundred and twenty gives a fine image of 

 Saturn, and sometimes one thousand diameters is useful, 

 though the lower power is to he preferred. This singular 

 peculiarity has always been apparent to me with all 

 telescopes in observing .Jupiter and Saturn. Saturn will 

 stand a much higher power than Jupiter, the latter 

 becoming vague and vapory with high powers. The 

 moons of Jupiter, however, readily stand one thousand, 

 giving clearly-defined discs with more or less stray Ught. 

 The newly-discovered fifth satellite does not stand mag- 

 nifying well and is lost with the higher powers. 



I have lately been measuring the dimensions of the 

 asteroids Ceres, Pallas, and Vesta. They present well- 

 defined measurable discs and are very satisfactorily observed 

 with one thousand diameters, and on a good night I would 

 much prefer this power. 



I think Mr. Ranyard's estimate of the powers that can 

 be used on the planets with the thirty-six-inch — forty or 

 fifty diameters to the inch— is, if anything, too high, 

 twenty or thirty would more nearly represent the values, 

 and for -Jupiter ten to fifteen, in my opinion, would be 

 nearer the mark. 



In comet observations much will depend on the comet. 

 A smaU comet, with a definite condensation, is best seen 

 with powers of two hundred and sixty or three hundred 

 and fifty, and sometimes with five hundi-ed and twenty and 

 seven hundred. It is surprising how some comets will 

 stand magnifying. If the comet is large and diffused it 

 had best not be observed with the thirty-six-inch at all, as 

 the great separating power of that instrument would diffuse 

 it into nothing. I remember one night Mr. Burnham and 

 I tried to observe a nebula which was quite noticeable in 

 the finder, and we finally gave it up in despair without 

 having seen the sUghtest trace of it in the thirty-six-inch 

 with any power. But for the planetary and small indefi- 

 nite nebulse the great telescope is pre-eminent. 



There is a very large eye-piece belonging to this instru- 

 ment that is very seldom used. The field lens is six inches 

 in diameter, and it will take in the entire moon. Whei-e one 

 would expect this to be specially useful — for faint objects — 

 it is very disappointing. The sky becomes whitish and 

 milky in it, and one thus loses a delicate faint object for 

 want of blackness of field. I tried comets 1889 I. and 

 1889 v., when very distant from us, with this eye-piece, but 

 could not see a trace of them on account of the brightness 

 of the field ; but with powers of three hundred and fifty and 

 five hundred and twenty I followed the first of these objects 

 over one hundred million miles beyond the orbit of Jupiter, 

 and the other I observed nearly a year after it was lost to 

 all other telescopes. A view of the Orion nebula through 

 this large eye-piece is, however, very fine. 



In reply to Mr. C. Robinson's letter published in 

 Knowledge for March, p. 64, I would remark that the 

 blue colour about Jupiter or a bright star in the thirty-six- 

 inch is much greater than in a good six-inch telescope. 

 The secondary spectrum is much more pronounced in the 

 thirty- six-inch than in the twelve-inch. 



In speaking of the Orion nebula, there is a feature in 

 connection with some of its details that I have never seen 



