596 



NA TURE 



[February 21, 1895 



of comet /■ iSii, Dr. Janssen fiorts that ihe intensity decreised 

 in a ratio between the fourth and sixth power of the distance 

 from the nucleus. 



Atmospheric Dispersion'. — The fact that the image of a 

 star as seen in a telescope is drawn out into a short vertical 

 spectrum, with the red end uppermost, was noticed as long ago 

 as 1729 by Bautjaer, 'and the effect of the difterence in colours 

 of stars upon refraction appears to have been indicated in a 

 general way by Lee in 1815. Even in small instruments this 

 at-nospheric spectrum is very noliceab'e in ihe case of bright 

 stirs at low altitudes, but, if necessary, it can be corrected by 

 means of a thin prism placed in front of the eyepiece, or by 

 employing an eyepiece of the form deviled by the lale Sir 

 t>eorge .■Xiry. In a recent paper {Monthly Notices R.A.S., 

 January), Dr. Kambaut points out the importance, in these 

 days of extreme accuracy, of introducing a correction for 

 atmospheric dispersion — according to the varying colours of 

 stars — more especially in connection with observations of two 

 stars in close proximity, as in measures of double stars, and 

 observations for parallax. The claims of this hitherto rather 

 neglected factor appear to be fully substantiated by a series of 

 measures at different hour angles of S Cygni, in which double 

 star the colours of the components are strongly contrasted ; 

 they "show clearly a systematic difference affecting the dis- 

 tance between them, of the sort, and in the direction, that 

 theoretical considerations indicate." Dr. Rambaut also shows 

 that the systematic differences deperding upon hour angle in 

 the measures for the parallax of o Cenlauri by Drs. Gill and 

 Elkin, which they corrected by empirical formula', are due to 

 a diffcrenc: in the mean refrangibility of the light of the star 

 and of the comparison stars. Further con5rmation is derived 

 from a re-discussion of the Dunsink observations on the 

 parallax of 61 Cygni, and the resulting value is corrected from 

 o"'46s to o"'400. 



.•\n ingenious method of measuring atmospheric dispersion 

 has been devised by M. Prosper Henry, and values determined 

 for different colours (N'atikf. vol. xliii. y. 4001. 



I 



Let me bring before you one of the most perfect pieces 

 of workmanship in the world constructed to investigate 

 the phenomena of the heavens. It is a photograph of the 

 Lick Oli'^ervatory, situated at an elevation of 4000 feet on 

 Mount Hamilton. Mr. Lick, the founder, was a very ambitious 

 man. He was, I believe, an hotel-keeper at San Francisco, but 

 however that may be, he has made his name immortal by help- 

 ing on the progress of mankind. I wish we had some hotels 

 like the San Francisco hotel in this country, and some Mr. 

 Licks, because then some Englishman might immortalise him- 

 self in the same way. This, then, is the magnificent locality in 

 which a great deal of the work that I shall have to refer to has 

 been done. The principal instrument of this great Observatory 

 is a refracting telescope having an object-glass three feet 

 in diameter, and a lube fifty-four feet in length. This is 

 practically the most important telescope in the world at the 

 present moment, and to give you an idea of the wonderfully 



THE SluYS PLACE IN NATURE.' 

 I. 

 AM anxious to give in these lectures a statement, as clearly 

 and as judicially as I can, of the discussions which have been 

 going on since these results were published, to show what 

 holes have been picked in the new views, and what new 

 truths may be gathered from the new work which has now been 

 brought to bear upon the old, so that as a result the |)lace I 

 have given to the sun among its fellow stars may be justified 

 or withdrawn. These lectures will be different from the 

 former ones, inasmuch as I then attempted to give you 

 a piece of quiet history of several regions of fact and know- 

 ledge which had been well surveyed and mapped, and had 

 become part and parcel of the common property of mankind. 

 But now I shall have, in considering the discussion, rather to take 

 you with me into the forefront of those who are fighting the 

 battles on the confines of the unknown. I have to bring you news 

 from the front, something like that which we arc promised 

 to-morrow or the next day from Port Arthur. I have to .show 

 how the battle is waging, who has lost, what positions have 

 4>een occupied, and what things new and true and beautiful 

 have been wrested from the unknown region ; and I am 

 the more anxious to do that because it enables me lo bring be- 

 fore you the enormous advantages under which such work is now 

 carried on ; advantages in that now, when any question is put 

 to any part of the heavens, we know that there are many good 

 workers employed under the best possible conditions to get the 

 particular information that we want ; besides these advantages, 

 in every branch of inquiry we find advances gigantic, marvellous, 

 almost beyond belief. 



I am sorry lo .say that in this work the centre of gravity of 

 theactivity has left our country and has gone out West. We have 

 to look toour American cousins for a great deal that wc want to 

 know in these matters, for the reason that now they not only have 

 the biggest telescopes, and most skilled observers, but also they 

 have been wiser than we — they have occupied high points on the 

 earth's surface, and thus got rid of the atmospheric diflicullies 

 under which we suffer in England, and especially in London. 



> t !*:» of a course of IrclurcA lo working men 



• rology during Novcmt>cr and December, 



tS^i 



;) 



I'ic. 4-— Spectroscope atLiclicd 10 the cyc-cnd of tlic I.ick Telescope. 



broad way in which the authorities have gone to work, 

 I need only state the loUowing fact. Some of you who 

 have been in an observatory may remember that it has some- 

 times been very dilVicult to get the observatory chair at the 

 right height, or in the right position, for observing a star or any 

 celestial body with any comfort. The Americans get oyer this 

 by simply rai-ing the floor. liy means of hydraulics the 

 enormous floor, some 80 feet in diameter, is moved up and down 

 with the chair. The i'nporlance of spectroscopic work has not 

 been lost sight of in the equipment of the Oi-servaloiy, and a 

 very powerful spectroscope can be used in conjunction with the 

 great equatorial for observing or photographing the spectra of 

 ihe various celestial bodies (I'ig. 4). 



One of the most important telescopes in Knglanil at present 

 is Dr. kobcrls" reflector, with which several majestic represen- 



NO. I 32 I, VOL. 5 1] 



