230 



KNOWLEDGE 



[OLTOincR 1, IHOG. 



object, and that it had been much used, and by no tender 

 hands, to cheer those on board in the long Arctic niglit, 

 was apparent. 



In conchision, let U3 heartily congratulate Dr. Nansen 

 and his brave corapauious on their wonderful success, not 

 only in rcachini,' the fartliost point north ever before 

 attained by man. but in bringing back ship and crew safe 

 and well, and with three years' provisions still on board. 



n. F. W. 



Hcttcrs. 



[The Editors do not hold tlipmselves responsible for the opinions or 

 statements of eorrespondents.] 



UOW TO OBSERVK Till'. I.VTKKIOR OK ONE'S OWN EVES. 

 To the EdUorx of Knowledge. 



SiEs, — In Kkowleixje for May, 1894, Vol. XVII., 

 p. 117, a letter appeared describing how a man may fee a 

 cataract on his own eye by looking through a pinhole in a 

 card held close to the eye. But there are many other and 

 some better methods of seeing this and any other markings 

 there may be, either in, on, or in front of the eye ; eyelashes 

 will bo seen if they are allowed to come in the way, or 

 drops of water on the surface, :ind musca^ vohtantes are very 

 clearly seen. Some methods of viewing the marks have 

 one advantage, and others have another. One of the best 

 is by looking at a bright star or distant lamp through a 

 telefcope, using a high power eyepiece very greatly out of 

 focus. Instead of a star, the sun or moon may be used, 

 especially if the aperture of the telescope be reduced to 

 very small dimensions. Or an eyepiece alone, without the 

 telescope, but keking through a pinhole diaphragm. Or 

 one may look through a microscope at a concave reflector 

 turned to tl e sky, the reflector being covered by a piece of 

 paper with a small hole in it. Any point of light close to 

 the eye will do, such as the sun's reflection in a small 

 drop of water. 



In some of these wajs tie maiks may be seen quite 

 sharply defined and very highly magnified ; but they are 

 necessarily boidered by diffraction lines and rings. By 

 moving the eye, one can distinguish by the changing 

 positions and apparent size of the marks the various 

 depths from the surface at which they are situated ; 

 and by suitable experiments one can measure these 

 distances when the focal length of the lens used is 

 known. I thus calculate — without allowing for the re- 

 fraction of the eye — that in my own case the principal 

 marks are at a depth of about 26 inch from the surface, 

 while the next in importance are at about 0-10 inch. These 

 measur(ments are cnly rough, but I have no doubt it would 

 be possible to make tlum with a high degree of accuracy. 

 If the lefi action of the eye were allowed for, the distance 

 of the marks from the surface would come out greater. 



I have made an estimate of the degree of magnification 

 obtained by some of the methods named. One of the 

 marks on the lens of the eye was found to be 0-017 inch 

 across ; and this was so magnified as to subtend an angle 

 of seventeen degrees or more. If such an object were viewed 

 at a distance of ten inches, which is considered a normal 

 distance, its diameter would be six minutes of arc. Its 

 magnification to a diameter of seventeen degrees is therefore 

 equivalent to viewing it with a magnifying power of one 

 hundred and seventy. The advantage of high magnification 

 is partly neutralized by the difBculty of keeping one's eye 

 steady, and the consequent great mobility of the image ; 

 also by the outlines being less sharp. 



It is very interesting to observe the changes taking place 

 in one's eyes from time to time, and to note how extremely 

 slow most of such changes are. T. W. Backhouse. 



TAUIABLE STARS. 

 To the Editors of KNOwi.KDfiE. 



Sins, — Chandler's Third Catalogue of variable stars has 

 appeared, and is an invaluable document involving an im- 

 mense amount of work and judicial calmness, both of 

 which have been bestowed upon it to the great credit and 

 honour of the author. 



Turning to the stars in which the present writer and 

 many of your readers have been interested during the past 

 two years, Mr. Chandler, in his notes, says of Mira o Ceti: — 



" S()(). Discovered by Fabricius, 159(i ; recognized as 

 periodically var. by Ilohvarda, l(iS8. Principal epoch 

 of elements in catalogue corresponds to Fp. 227 of Arge- 

 landcr. Periodical terms given in Second Catalogue are 

 here suppressed. Altboagh they are undoubtedly real, 

 complication with other unknown terms makes the 

 accurate prediction of phases at present impossible. 

 9"' 1 foil. 7^ 7, 10" N." 



This satisfies my contention, and shows that the dis- 

 cussion on Mira in Knowledge last year was both timely 

 and useful — in good saason and with good cause. 



It is remarkable now, at the close of the third 

 centenary of its discovery, that such an announcement 

 should be made, and the question naturally arises : Has a 

 new term come in, or have the observations been more 

 frequent and careful — the observers more numerous and 

 self-relying '? No doubt new blood, fresh thought working 

 beyond the shadow of great minds, away from the in- 

 fluence of authority, is useful, if not essential to devilop- 

 ment and progress. Be this as it may, Mira the Wonder 

 sinks to her third centennial minimum a greater wonder — 

 a deeper mystery — than ever. 



Before quitting this subject, it may be useful to say that 

 full weight is not given by the writer to hi:? cstiuiate of 

 the light of Mira, January 19th (Knowled iE, Jun"). The 

 conditions were not quite normal. 



In reference to R Leonis, which has also been mentioned 

 in these notes, Mr. Chandler says : — 



''3493. The periodical inequality of the elements in 

 the Second tiatalogue certainly exist. But the observations 

 of the last few years show that it is comjlicated with 

 other unknown terms. Until the law is developed by 

 future observations, it seems best to use only the uniform 

 period of the catalogue for prediction of phases." 



In the last eight years the errors in miximum of this 

 star have been as follows : — 



Mr. H. M. Parkhurst has made no report on this star 

 as yet. The present writer's figures, which have already 

 appeared in Knowledge, are, for IB'.U, 1895, and 189(1 : 

 — 22'\ - 42'', and — 40' respectively. The change, so 

 sudden and so definite, is remarkable and suggestive of a 

 new force of an unknown nature. And it will hardly 

 escape attention that while a retarding force has operated 

 on one of these stars, an accelerating force has acted on 

 the other, and in nearly equal times ; one being behind 

 about as much as the other is ahead of time. 



Of R Scuti, 0733, Mr. Chandler says: "Large irregu- 

 larities." " Argelander found bright and faint minima, 

 usually alternating, and this has been confirmed by all 

 subsequent observers." lie states the main period and 

 gives no point of departure. A well-known observer writes 

 me, "No elements yet devised will fit this star long." 

 The maximum of R Scuti is due 2()th .July ; but a week 

 ago it was the faintest star in the little group and was 

 close to minimum. 



