224 



KNOW^LEDGE ♦ 



[March 13, 1885. 



predicates of many phenomena — food, pain, danger, &c. With ns 

 the case is, of course, very much stronger. Each of us takes such 

 a distinct, individual view of certain thinf;s (not mere opinions or 

 tastes, but impressions as to what phenomena really are before us), 

 as to have originated the proverb, " Quot homines, tot sententiso." 

 Tet, in spite of this almost infinite divergency, we have the identical 

 testimony of billions upon billions of men that certain facts are pre- 

 sented to the senses of all in the same way. Now, what antecedent 

 probability is there that each Ego should conceive the properties of 

 matter exactly the same as all the others, unless those properties 

 are real ? Is it not so improbable as to be untenable to suppose 

 that the whole race, differing on so many subjects of thought and 

 belief, should yet be at one as to certain common phenomena, unless 

 those phenomena are the manifestation of objective existence ? 



Hallyakds. 



THE NEW CHEOMATICS. 



[1632] — The theory of light would, I have reason to believe, be 

 considerably advanced by a more carefal and wider study of the 

 phenomena classed as the oculiir spectra. It ought to be more 

 generally known that the true science of the harmony of colours is 

 based on the study of these phenomena, and also that everyone 

 ca,rries about with him the necessary appliances for experimenting 

 with them— namely, e.yes. But, as excessive use of the eyes for 

 that purpose is injurious, it would be well if a great number of 

 investigators would each experiment a little, and so divide the 

 labour. The ocular spectra supervening on sun-daze are not onlv 

 very interesting when examined, but present peculiar features tha't 

 still stand in need of explanation ; for instance, any one of the 

 sequent spectra changes colour if its field be changed from light ro 

 dark, or i-icc rcrsi. A red spectrum in a light field becomes a vivid 

 green m a dark. So that the same impression, the same image, 

 alternately ajjpears of the contrasting, or complementary colour. 

 And if the initial impression bo sufficiently strong, the experiment 

 may be several times repeated with the same spectrum. This fact, 

 moreover, appears to me to furnish conclusive evidence that Dr! 

 Young's three nerve hypothesis is erroneous. W. Cave Thomas. 



MEANTIME SUNDIALS. 



[1633]— In the Emjlisli Mechanic for Aug. 19,1870, Vol. XI., 

 page 523, col. 2nd. at the end of a reply to query 4131, occurs the 

 following passage :— " It is not generally known that dials (and 

 those of elegant forms) may easily be made to .show clock-time, 

 exact to a quarter minute, throughout the year, and even to a 

 second (except in a few days at midwinter, and fewer at mid- 

 summer), by having two gnomons, used alternately for nearly half- 

 a-year each. I propose this problem to the ingenuity of some of 

 your readers. — E. L. G." 



I am not aware that any of the readers so appealed to sent a 

 satisfactory solution of the problem, and the name of the proposer 

 of it has since ceased to appear as a signature in the correspondence 

 columns of the B. M. As, however, I see that he sends interesting 

 letters to K.nowledge, I venture to take this means of asking him 

 if any description of these sun-dials has been published, and if so, 

 where, and if not, to suggest to him the desii-ability of either 

 publishing the details of their construction, or of causing them to 

 be made for sale, which I am not aware that they have been. 



My own solution of the jiroblem would be as follows, but from 

 from what " E. L. (}." says on pp. 597 and 5iJ8 of the same volume, 

 and I think elsewhere in the E. M., I infer that it is not his 

 solution. Let marks indicating hours and their subdivisions be 

 made on the inner surface of a part of a ring extending no farther 

 than is needed to contain the sunlight hours of the longest day, the 

 hour marks being fifteen degrees apart, or at the rate of twenty- 

 four hours to the circumference. Let this be fixed with 

 its axis parallel to that of the earth, and let the gnomon 

 be a solid of revolution, the generating curve of which 

 IS such that, when the gnomon is suitably placed with its axis coin- 

 cident with that of the other part of the dial, one side of it shall 

 just hide the sun from the 13 o'clock mark at mean noon daring 

 the period from one solstice to another, when it is to be exchanged 

 for the other gnomon, which must be so shaped on the same prin- 

 ciples as to act in tlie same way during the other half-year. The 

 objection to this plan, in my mind, is that there would be a risk of 

 reading off the position of" the shadow of the wronri side of the 

 gnomon, and so making a mistake as to the time, and I do not see 

 any way of obviating this without diminishing the number of hours 

 during which the dial would be available. A suitable screen, ex- 

 tending from the gnomon to either the mark at six or that at 

 eighteen, would cure this defect, but would also preclude the dial 

 from being used in summer for more than about twelve hours of 

 the day, unless, indeed, the gnomon were made very thick, and 



would detract from the elegance of which " E. L. G." speaks. The 

 graduations must not consist of lines, as in the common snndial, 

 but ]ioints, which could be indicated by the intersections of a line 

 running through them all with short cross lines having suitable 

 numbers at their ends to indicate the hours. If the ends of the 

 graduated arc are sufiiciently thin in the direction of its axis, there 

 will only be a very few days at the equinoxes during which their 

 shadows will interfere with that of the gnomon. D. L. X. 



A CURIOUS LUNAE PHENOMENON. 



[1034] — Observing the moon on the 19th ult., with a SJ-in. re- 

 fractor, I noticed that the small crater inside Hercules, instead of 

 being tilled with black shadow, shone with a dull, deep-red colour. 

 The power I was using was 100. I changed it for ISO, and again 

 for 250, without any alteration in the peculiar appearance of the 

 crater; and 1 also compared, or rather contrasted, the shade of the 

 crater with every shade I could find, which ordinarily it would have 

 resembled, especially that of every crater, similar in size, shape, 

 and position. The time was between seven and eight p.m., and the 

 crater near the morning terminator. 



The hue was quite obvious, and seemed to catch the eye imme- 

 diatel}', even in a large field. 



I turned the telescope to the same spot on the next evening (20th 

 tilt.), but could perceive no abnormal appearance. 



I shall be glad to hear if any (^ther of your readers have noticed 

 this phenomenon, and if any explanation can be offered. 



William Ge.\y, Jin. 



LUNAR GLOW. 



[1635] — Whilst visiting Eome friends at Blairmore, Loch Long, 

 Argyleshire, on Saturday, 28th ult., I observed rather a curious 

 lunar phenomenon. While the moon rose above the hills opposite 

 Blairmore, and appeared in fall view, the sky below, and spread 

 for a considerable distance, had a sort of dim red or rose- 

 colour appearance, something like the after-glows that appeared so 

 frequently autumn before last. I observed this about 7.15 o'clock, 

 and it lasted till near 10 o'clock, appearing very bright between 

 8 and 9 o'clock. Can you explain this curious phenomencn, or has 

 any of your other readers observed this. I had another friend 

 walking with me at the time, and pointed this out to him. 



Jon\ A. Stewart. 



[Records exist of the observation of this phenomenon during 

 the last two years ; but, at this moment, I am unable to lay my 

 hand uijon any of them.] — Ed. 



THE PHILOSOPHY OF ARGUMENT. 



[1636] — As a constant reader of your journal, would yon allow 

 me space for a few remarks on this subject, which, I think, comes 

 within the scope of the principles you advocate, and which are sug- 

 gested by one or two remarks wliich have appeared lately in the 

 letters of correspondents and in other paragraphs of Knowledge. 



There is nothing more delightful than the thought that all know- 

 ledge can be brought into the calm light of a true philosophy, and 

 there weighed and tested, approved or rejected, as truth permits. 

 This is not only a delightful, but a true theory. But, alas! like a 

 good many good theories in this world of ours, it but too often 

 breaks down in practice. Nevertheless, it may be worth while to 

 endeavour to make out how and where it breaks down, and vrhy ; 

 to see whether it is altogether impossible to reduce to first prin- 

 ciples, and to systematise the way in -whicli this theory breaks 

 down. 



To plunge at once in mcdias res, and to illustrate this matter by 

 an example, I make an assertion. You meet that as.sertion, not by 

 a counter eyo dico, nor by a refutation of my facts, or logical deduc- 

 tion from them, but by the counter assertion that I am but ad- 

 vancing as .arguments deductions from my own sensations or my 

 own peculiar views of lite. 



Let us take this as an example of philosophic argument which 

 sometimes is pursued, and see whether there is not something in it 

 which is worth devoting a little attention to. 



A priori every man's opinion is of as much account as every 

 other man's, but, when he has shown himself to be unworthy of 

 credit by self-refntafion, his opinion ceases to be of value. But the 

 question I sliould like to raise is this. In what other way can he 

 render himself from a philosophical point of view unworthy of 

 credit, ea'cepf by self-refutation ? 



The reply to this may be, "When he controverts doctrines whicli 

 by a consensus of opinion of philosophers ought to be maintained." 

 But this is not philosophy. As a matter of fact, there is no such 

 consensus of opinion. And the reply, to be put in a philosophic 

 form, should be this, " When he controverts doctrines which, in my 



