March 5, 1885 J 



NA TURE 



407 



already known, with some additional facts obtained by 

 the observation of himself and his brother missionaries. 

 There is a map and a few good illustrations. 

 Three Months in the Soudan. By Ernestine Sartorius. 



(London : Kegan Paul and Co., 1885.) 

 Mrs. Sartorius spent most of her three months in 

 1883-84 at Suakim, of which her husband, Gen. Sartorius, 

 was Commandant. Her book deals chiefly with the events 

 which culminated in the disaster of El-Teb. It is mostly 

 a pleasant, gossipy record of the daily life of the town, 

 and of the alarms created by the attempted raids of the 

 rebellious natives in the district around. It affords a 

 good idea of the character of the town and its immediate 

 surroundings. 



Lectures on Agricultural Science and other Proceedings 

 of the Institute of Agriculture, South Kensington, 

 London, 1883-84. (London : Chapman and Hall.) 

 Tin? volume contains abstracts of lectures delivered by a 

 considerable number of well-known authorities upon agri- 

 cultural matters. Mr. Carruthers and the late Prof. 

 Buckman give their experiences upon grasses and farm 

 seeds ; Prof. Wrightson has a paper upon land drainings ; 

 dairy management and farm crops are treated of by Pro- 

 fessors Huldon and Fream and Mr. Bernard Dyer ; Mr. 

 Henry Woods contributes lectures upon Southdown sheep 

 and ensilage ; while Mr. Warrington has a contribution 

 upon the nitrogenous matter in soils ; and Mr. Worthing- 

 ton Smith gives some good observations upon corn 

 mildews. The names of the authors of the various lectures 

 are a sufficient guarantee of their soundness and worth. 



LETTERS TO THE EDITOR 

 [ The Editor doesnot hold 'himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of rejected manuscripts. 

 No notice is taken of anonymous communications . 

 [ The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts.] 



Sir William Thomson's Baltimore Lectures 



As it is possible that some of your readers may have obtained 

 copies of the Papyrograph Report of my Lectures on " Molecular 

 Dynamics," delivered at Baltimore during October 1SS4, I 

 should be obliged by your giving publicity to the following 

 corrections : — 



Page 34, lines 18 and 19. — Delete " we may call it a dynamox 

 but not a paradox." I have no recollection nor can I imagine 

 what the word was that I suggested as more logical than 

 " parad' 



Page 59, line 14. — For " Distortional " substitute " Condensa- 

 tional." 



Page 296. — In the two expressions for if*, given in equation 



(171. insert "tan 1" before „~ ' ; also, in the expres- 



h- + 1 

 sions for " tan e " and " tan c, " of equation (20) insert " tan ;' " 



before""'" "' 

 ," J - 1 

 The formula from which these expressions are deduced is 

 correctly given at the foot of pape 295. 



Page 296. — In line 13 from the top of the page, and in the 

 left hand members of equations (19) and (21), for "a" and 

 " o>[ " read "«'' and "a, " respectively. 



William Thomson 

 The University, Glasgow, February 27 



Civilisation and Eyesight 

 It would take too much of your space to discuss at length the 

 theoretical limit of resolving-power as depending upon aperture. 

 The reader may be referred to some papers in the Philosophical 

 Magazine for 1879 and 1880, where he will also find references 

 to the work of other investigators. I will only say that (as indi- 

 cated by the word fairly in my statement) resolution admits of 

 various degrees. Doubtless a practised observer would judge a 



star to be double whose components subtend a decidedly smaller 

 angle than two minutes, but he would not see them separated. 

 I purposely rather understated the case. The higher the visual 

 power of civilised men, the less room is there for savages with 

 eyes of equal aperture to surpass them. 



With respect to my short-sightedness in a bad light, I shall be 

 glad if you will publish the accompanying two short papers from 

 the Cambridge Philosophical Proceedings. They will show how 

 I was led to make the discovery. Rayleigh 



"The Use of Telescopes on Dark Nights." By Lord 

 Rayleigh. (From the Comb. Phil. Proc, March, 1882.) 



In Silliman's Journal for 1S81 Mr. E. S. HoHen, after 

 quoting observations to a like effect by Sir W. Herschel, gives 

 details of some observations recently made wi.h a large tele- 

 scope at the Washburn Observatory, from which it appears that 

 distant objects on a dark but clear night can be seen with the 

 telescope long after they have ceased to be visible with the naked 

 eye. He concludes, " it appears to me that this confirmation 

 of Herschel's experiments is important, and worth the attention 

 of physicists. So far as I know there is no satisfactory explana- 

 tion of the action of the ordinary night-glass, nor of the similar 

 effect when large apertures are used." 



It is a well-known principle that no optical combination can 

 increase what is called the "apparent brightness" of a distant 

 object, and indeed that in consequence of the inevitable loss of 

 light by absorption and reflection the "apparent .brightness" is 

 necessarily diminished by every form of telescope. Having full 

 confidence in this principle, I was precluded from seeking the 

 explanation of the advantage in any peculiar action of the tele- 

 scope, but was driven to the conclusion that the question was 

 one of apparent magnitude only, — that a large area of given 

 small "apparent brightness" must be visible against a dark 

 ground when a small area would not be visible. The experi- 

 ment was tried in the simplest possible manner by cutting crosses 

 of various sizes out of a piece of white paper and arranging them 

 in a dark room against a black background. A feeble light 

 proceeded from a nearly turned-out gas-flame. The result 

 proved that the visibility was a question of apparent magnitude 

 to a greater extent than I had believed possible A distance 

 was readily found at which the larger crosses were plainly 

 visible, while the smaller were quite indistinguishable. To 

 bring the latter into view it was necessary either to increase the 

 light considerably, to approach nearer, or lastly to use a tele- 

 scope. With sufficient illumination the smallest crosses used 

 were seen perfectly defined at the full distance. 



There seems to be no doubt that the explanation is to be 

 sought within the domain of physiological optics. It has 

 occurred to me as possible that with the large aperture of the 

 pupil called into play in a dark place, the focussing may be very 

 defective on account of aberration. The illumination on the 

 retina might then be really less in the image of a small than in 

 the image of a large object of equal "apparent brightness." 



"On the Invisibility of Small Objects in a Bad Light." By 

 Lord Rayleigh. (Fr..m the Cambridge Phil. Proc, Feb., 1883.) 



In a former communication to the Society (March 6, 1882) I 

 made sme remarks upon the extraordinary influence of apparent 

 magnitude upon the visibility of objects whose "apparent bright- 

 ness " was given, and I hazarded the suggestion that in conse- 

 quence of aberration (attending the large aperture of the pupil 

 called into operation in a bad light) the focussing might be 

 defective. Further experiment has proved that in my own case 

 at any rate much of the effect is attributable to an even simpler 

 cause. I have found that in a nearly rlark room I am distinctly 

 shortsighted. With concave spectacles of 36" negative focus 

 my vision is rendered much sharper, and is attended with in- 

 creased binocular effect. On a dark night small stars are much 

 more evident with the aid of the spectacles than without them. 



In a moderately good light I can detect no signs of short- 

 sightedness. In trying to read large print at a distance I suc- 

 ceeded rather better without the glasses than with them. It 

 seems therefore that the effect is not to be regarded as merely 

 an aggravation of permanent short-sightedness by increase of 

 aperture. 



The use of spectacles does not however put the small and 

 the large objects on a level of brightness when seen in a bad 

 light, and the outstanding difference may still be plausibly attri- 

 buted to aberration. 



Mr. Carter's recent paper on "Civilisation and Eyesight '> 

 has called up interesting remarks from Lord Rayleigh and Mr 



