400 



♦ KNO\VLEDGE 



[May 8, 1885. 



colonr — provided that our rate of approacli bears au a[)prociable 

 ratio to the cnormoTis velocity of light. Tlic change of colour is 

 towards the red end of the colour-gamut if wo are receding, 

 towards the violet end if we are approaching. Honce has been 

 derived the now celebrated method of determining rates of approach 

 and recession of stars, star hurricanes, &c., viz., by displacement of 

 some known line of the spectrum — a method in which I take much 

 interest, seeing that I published a full, clear, and particular account 

 of the method a considerable time before it was known that any 

 attempt had been made to apply the method, or oven that the 

 method had occurred to any one. Some months after my article 

 on the subject (in Fra.-icr's ilaguzinc) Father Secchi and Dr. 

 Huggins severally announced the successful application of the 

 method, and cnrlier than that, in fact immediately after my article 

 appeared, Dr. Huggins informed rae privately that he had been 

 planning to apply the very method I had described. But, if any 

 value attaches to tjie first public description of a method of re- 

 search, afterwards applied successfully, I believe I can claim so 

 much as that in this instance. Doppler's idea, of course, w.as 

 valueless, as I pointed out ; because in the case of a body like a 

 star emitting rays of all the colours of the rainbow — besides invi- 

 sible rays both on the violet and the red side of the spectrum — 

 there would be no .apparent change of colour, let the motion of 

 approach or recession be ever so rapid. — R. P.] 



THE RUDDY ECLIPSED MOON. 



[1697] — I am pleased to see that Mr. Proctor has discussed one 

 of my small astronomical heresies (Knowledge, p. 339), .although 

 he has handled it rather roughly. However we may differ in our 

 conclusions, we are perfectly agreed in our object — the elucidation 

 of truth. A man who has only one original idea, one dearly, 

 beloved theory, usually regards everybody who disputes it as a 

 personal enemy, and defends it savagely as a matter of personal 

 honour. As neither Mr. Proctor nor myself are thus limited, we 

 can mutually afford to discuss any one or another of our specula- 

 tions or conclusions without personal offence, and to speak quite 

 plainly in doing so. In this case we are widely at variance. I am 

 as fully satisfied that he is wrong, as he is evidently convinced of 

 my error. 



In tlie first place, Mr. Proctor says that " Professor Langley's 

 refined experiments with his delicate heat measurei's (the ' bolometer,' 

 I presume,) indicate for the moon's surface, even when she is full 

 or exposed to the full blaze of sunlight, a temperature many 

 degrees below the freezing-point." If the bolometer tells such a 

 story as this, I can only reply, " So much the worse for the bolo- 

 meter." There must either be " something wrong with the sun," 

 or something wrong with the moon, or something wrong with the 

 bolometer, unless we assume that the laws of thermal radiation, as 

 observed on the earth, do not prevail at the moderate lunar distance 

 beyond it. 



It has been proved, not by one experiment, nor one heat measurer, 

 but by every experiment ever made, intentionally or inevitably, 

 that a solid body '* exposed to the full blaze of sunlight " is heated 

 greatly beyond the freezing-point, and to believe any heat measurer, 

 however delicate, that told such a tale as Mr. Proctor recounts is 

 comparable to the faith of a member of the Soudan expedition who, 

 having a newly-invented thermometer that registered many degrees 

 below freezing when exposed to tlie full blaze of the summer sun 

 at Suakim, should believe that thermometer in preference to the 

 evidence of his own senses. 



There is one possible condition of a solid bodj', situated like the 

 moon, receiving the full blaze of solar radiation without becoming 

 correspondingly heated — viz., that it be a perfect reflector. If the 

 moon were a ball of burnished silver it would reflect 90 per cent, of 

 the solar rays, and the remaining 10 per cent, of solar energy would 

 operate in raising its temperature above that of space ; bur, then the 

 moon v.-ould be more dazzling to our vision than the bit of looking- 

 glass by means of which the naughty boy on the sunny side of the 

 street prevents the good old gentleman from reading his newspaper 

 at the opposite window. 



Mr. Proctor next proceeds to tell us that the ruddy glow is pro- 

 duced by the transmission of " the sun's very own rays," and to 

 state the amount of those which reach the moon by means of 

 atmospheric refraction. We are agi-eed concerning the maximum 

 thirty-five minutes of arc of entering refraction, and the additional 

 thirty-five minutes of emerging retraction, making 1° lu' altogether 

 at sea-level ; but when he assumes that these rays, thus diverted 

 from their original course, will proceed to the moon as though they 

 came in approximately parallel course from the sun, I go with him 

 no further. 



I maintain that after such refraction they become converging 

 rays proceeding to several foci in space beyond the earth, then 

 crossing and diverging. If the sun were a point of light and our 



atmosphere of uniform sea-level density throughout and achro- 

 matic, these rays would form one cone and have one focus corre- 

 sponding in position to the apex of an isosceles triangle with base 

 = earth's diameter, and interior angles at base = 88' 50' (taking 

 Mr. Proctor's figures, or 88° 38' 1-1", taking Sir John Ilerschel's). 



But the sun not being a luminous point, and our atmosphere not 

 being uniform, nor corrected like an achrom.aticlens, there must be 

 a multitude of such cones and a corresponding distribution of foci. 

 The limits of the space in which these foci occur are clearly worked 

 out in Sir John Herschel's " Outlines of Astronomy," to which I 

 originally referred as an exposition of the "usually-accepted theory," 

 and which Mr. Proctor says I *' evidently misunderstand." Whether 

 it is I or Mr. Proctor who misunderstands, is indicated by the fact 

 that his estim.ate of the result is profoundly at variance with that 

 of Sir John Herschel, while mine is in perfect harmony with it. 

 Sir John Herschel concludes his demonstration of the paths of the 

 refracted rays by saying " it is evident that the totality of light 

 thus thi-own into the shadow, is to that which the earth intercepts, 

 as the area of a circular section of the atmosphere to that of a 

 diametrical section of the earth itself, and therefore, at all events, 

 but feeble." 



I said the same, my words being, "representing the earth by a 

 globe of one foot in diameter, it (the ring of light) would be shown 

 in projiortion by a line of light j-^Vi; "f ^ foot, or ^J ^ of an inch, 

 about the thickness of the paper on which this is printed." Mr. 

 Proctor's conclusions are extravagantly at variance with these. 



He says that " if the sun were a bright point just beyond the 

 earth's centre, as looked at from the moon," he would be seen as a 

 ring all round the earth. I (following the demonstration of Sir 

 John Herschel) maintain that in such a case he would not be seen 

 there at all, as the focus of the rays from this point would fall 

 between the earth and the moon, as shown in Sir John Herschel's 

 diagram, and beyond this they would diverge and overshoot 

 altogether that " single point on the earthward lunar face " which 

 is Mr. Proctor's lunar standpoint. 



According to Mr. Proctor's reasoning and diagrams, an eclipse of 

 the moon by the earth's shadow cannot occur at all. He shows 

 that the whole of the sun is uplifted into view from the lunar stand- 

 point ; that the sun is wholly and fully shining upon the moon 

 without being hidden at all by the solid body of the earth. He 

 thus presents us with a complete reiuctio nd ahsurdum of his own 

 argument, for if such were the case the only eclipse the moon could 

 suffer would be due to the shadow of our atmosphere. If the 

 whole of the sun, as seen from the moon, were optically trans- 

 ferred from behind the earth to its circumference, and there spread 

 out as a ring in the manner stated, and so plainly shown in Jfr. 

 Proctor's diagrams, the shadow of the earth, instead of being thrown 

 upon the moon, would be simply projected towards its own centre. 

 The effect on the moon would be simply to display an alteration of 

 the shape of the sun — a conversion of the solar disc into a solar 

 ring, radiating the same amount of light as the disc, minus only 

 that obstructed by our atmospliere. AU his diagrams and reasoning 

 show this, even the details of the distorted spots. 



In his calculation of the amount of light thus displayed, he falls 

 into a fallacy which surprises me, seeing thac it is purely mathe- 

 matical. The figures on the second column of page 3-tl affirm that 

 the amount of sunlight transmitted by the annular whole sun 

 (assuming that one-half is absorbed in passage) is y^Vir ^^ ^^^ 

 ordinary sunlight. This conclusion is reached by comparing the 

 comparative area of the annular or distorted sun to that of the 

 solar disc ; but Mr. Proctor surely undei-stands that if the ivliole 

 nun is represented by that ring in the manner he describes, the 

 intensity of the luminosity of the ring, as compared with that of 

 the solar disc, will vary inversely with the apparent area of the 

 ring as compared with the apparent area of the disc ; or, otherwise 

 stated, the total luminosity of the ring will be equal to the total 

 luminosity of disc (viewed under like circumstances) whatever be 

 their comparative areas. Therefore, with such a ring displaying 

 the vhoJe sun to the moon in the manner stated, the eclipsed moon 

 would only differ from the full moon by the loss of light due to 

 atmospheric obstruction. 



This reply has extended so far that I must not describe in detail 

 the observation I made on the eclipse of the moon October 4 last, 

 when the "poor thing" did not show any ruddy glow at all, but 

 only a faint, dirty grey illumination, totally different from that of 

 the eclipse of August 23, 1877, which I observed during my walk 

 from Castlebar to Ballina. 1 may, however, mention tliat in 1877 

 the glow "was uniform over the whole face of the moon, and 

 gradually faded during the totality, as it should do if the moon 

 itself were cooling down by radiation, while the grey twilight of 

 1881 was one-sided at first, then nearly disappeared when the moon 

 was in the centre of the shadow, and reappeared on the opposite 

 side as the moon approached the opposite limit of the shadow. 

 This is exactly what should occur according to Sir John Herschel's 



