June, 1913. 



KNOWLEDGE. 



239 



extraordinary that Professor Lowell with his twenty-four inch 

 refractor has apparently failed to do so ? The following 

 passage, " areographers forget . . . that what is clear and 

 sharp on such a small disc so far off ought to be represented 

 as exceedingly vague on drawings three inches in diameter," 

 possibly supplies an explanation of the apparent telescopic 

 blindness of certain observers. This is what the statement 

 amounts to : You may see a system of sharply defined lines 

 on the disc of Mars, but when representing them in a drawing 

 you must give them an uncertain foggy appearance. If you 

 are sketching a distant balloon, and can quite distinctly see all 

 the ropes as very fine sharp lines against the sky, you must 

 represent them in your sketch as vague, uncertain bands. 



I am afraid that I must, with all respect, disagree with the 

 following passage on point of fact : " The student who passes 

 many consecutive hours in the study of Mars with medium- 

 sized instruments is liable to catch rare glimpses of straight 

 lines, single or double, generally lasting about one quarter of 

 a second." I have myself studied Mars with a nine-inch 

 reflector, and have seen and held for considerable periods 

 several of the larger " canals." The sensitiveness of observers 

 varies very much, and also the visibility of " canals " is not 

 always the same. It is a mistake for any one observer, how- 

 ever practised, to take his own experience and apply it to 

 observers in general. Bearing in mind the great fluctuations 

 in visibility of the markings on Mars — and indeed markings 

 which come out strongly at one season will be practically 

 invisible at another — the contrasts shown in Figures 190 to 

 193 lo?c some of their point. Mr. Antoniadi pours great 

 ridicule on the advocates of comparatively small telescopes. 

 He discusses this by no means settled question as if aperture 

 were the only thing to be taken into consideration. In order 

 to counteract the effects of the secondary spectrum, the 

 greater the aperture of a telescope the greater must be its 

 focal length. With telescopes of very great aperture it has 

 proved, so far, a mechanical impossibility to make the focal 

 length sufficiently great. In the perception of the existence of 

 faint stars and strands of nebulosity, for which these tele- 

 scopes of great aperture are especially suited, exact definition 

 is not important ; but in the case of planets, perfect definition 

 is of more importance than light-grasp. 



Definition depends, too, not only on the telescope, but also 

 to a' very large degree indeed on climatic conditions, which 

 Mr. Antoniadi has not taken into account in his article. 

 Professor Lowell has, at the Flagstaff Observatory, probably 

 the finest possible equipment for planetary work. The 

 climatic conditions at Flagstaff are undeniably better than 

 those enjoyed by any other observatory. His twenty-four- 

 inch refractor is, as refractors go, optically perfect. In 

 planetary work his best results are obtained with the aperture 

 stopped down to eighteen inches. The results obtained by 

 Professor Lowell in planetary work are a testimony to the 

 advantages of his observatory. His photographs of the 

 planets, notably of Saturn and Jupiter, are, I believe, 

 admitted to be unequalled. Professor Lowell is the greatest 

 advocate of the theory of the artificial origin of the canals. 

 He has succeeded in obtaining photographs of Mars on which 

 canals appear. Yet Professor Lowell, who, with every 

 advantage and equipment, has made the study of Mars the 

 chief work of his life, and who certainly has more right to 

 speak upon the subject, as an observer, than any other 

 astronomer, is not mentioned by Mr. Antoniadi in his article. 



84, Dartmouth Road, J. E. MAXWELL. 



Brondesbury, N.W. 



THE DOUBLE (AND BINARY) STARS. 

 To the Editors of " Knowledge." 

 Sirs,- — There are some statements in Mr. Henkel's paper 

 in April " Knowledge " that require qualification. He writes: 

 " T . he c ? iscs ' of stars > tnat is ) seen by the naked eye, being 

 optical illusions, are effects of irradiation." Now, no one sees 

 " discs " with the naked eye at all ; as is well known, they are 

 only seen when magnifying power is applied. Moreover, they 

 are not effects of irradiation at all, but of " diffraction " — inter- 

 ference phenomena, in fact. 



He says that Herschel said, in his own words, " he went 

 out like Saul to seek his father's asses, and found a kingdom." 

 It was not Herschel who said this, but Schwabe, and it was 

 said in relation to the discovery of the periodicity of sun-spots. 



Mr. Henkel says : " Perhaps as many as twelve thousand 

 such couples are known," meaning double stars. There is no 

 "perhaps." Burnham's list contains thirteen thousand six 

 hundred and fifty-five. Re catalogues, neither Struve's' 

 " Mensurae " nor Lewis's Catalogue is now the standard 

 authority, but Burnham's. 



There are other too positive statements upon matters where 

 facts are doubtful and opinions differ, but I pass over merelv 

 doubtful assertions. EDWIN HOLMES. 



THE FOURTH DIMENSION. 

 To the Editors of " Knowledge." 



Sirs, — I have no desire to quarrel with Mr. Henkel's 

 tentative "working definition" of "reality" as "that which 

 exists independently of any human mind perceiving it." But 

 I would qualify "reality" so defined as "objective," i.e., 

 universally valid ; for we obviously cannot deny reality to our 

 individual ideas, though we may usefully distinguish between 

 this reality, which is subjective, i.e., only true for the individual, 

 and that which is " objective," as defined above. And I 

 would add that, in order to conceive of a world existing 

 independently of any human mind perceiving it, we must 

 conceive of it as existing in some other mind. So that 

 Mr. Henkel's definition does not answer my objection. 



Concerning the objects of vision, Mr. Henkel's position is 

 not easy to state in philosophical language. He appears to 

 argue that our visual percepts arise from the same causes as 

 our tactual percepts, and that in order for any such cause to 

 give rise to a two-dimensioned visual percept, it must be 

 capable of giving rise to a three-dimensioned tactual percept. 

 This may, indeed, be true. But its truth or falsity does not 

 affect the argument, because in either case the fact remains 

 that the objects of vision (i.e., the visual percepts) are two- 

 dimensioned. And Mr. Henkel's objection to my theory of 

 the fourth dimension was that it was built upon the assumption 

 of the existence of objects having less than three dimensions, 

 which Mr. Henkel said did not exist. But vision does 

 immediately acquaint us with two - dimensioned objects. 

 Whether the causes of our visual percepts are capable of 

 producing three-dimensioned tactual percepts is a question 

 which does not affect this fact. The existence of such causes 

 is an inference and is not given immediately by experience. 



But, indeed, Mr. Henkel admits the existence of a one- 

 dimensioned object of experience, i.e., time, and thus seems 

 himself to demolish his objection to my theory. 



The Polytechnic, H. S. REDGROVE. 



Regent Street, W. 



P.S. — As I have already intimated, I do not intend to dis- 

 cuss the question further with Mr. Johnston, but I must correct 

 two misstatements in his letter. In the first place, my letter 

 in your February issue was not written, as Mr. Johnston says, 

 to express surprise that he had not gone into more detail in 

 his former letter, but to point out that as he had not taken, and 

 would not take, the trouble to acquaint himself with the 

 arguments upon which my theory was based, he was incom- 

 petent to criticise it. In the second place, Mr. Johnston is in 

 error when he says that in my letter in your March issue I 

 gave my " argument in more detail, so it is possible for more 

 criticism to be made." What I merely did in that letter was 

 to answer objections raised by Mr. Henkel. If Mr. Johnston 

 really wanted my argument in more detail, he could have 

 found it at the references I gave. If he had done that it 

 might have been possible for him usefully to criticise the 

 theory in question. 



PROBLEMS OF LIFE AND REPRODUCTION. 



To the Editors of " Knowledge." 



Sirs, — I hold that it is rank impertinence of an author to 

 controvert the judgment of the critic. It is, then, merely a 

 matter of interest to note that after about a dozen criticisms 



