1855.] 



THE SOLAR ECLIPSE OF 1854. 



185 



light coming from the object, since the same degree of illu- 

 mination which, under certain conditions, will render the 

 body visible, will under other circumstances, fail to produce 

 any sensible impression. Thus, for example, the lumiere cen- 

 dree, or earth-shine on the moon, which is perceptible by the 

 naked eye after sunset, cannot be detected with the aid of a 

 telescope while the sun is above the horizon. This difference 

 is due to the dispersive power of the atmosphere. When the 

 sun's rays fall upon any one of the minute particles of the air, 

 they are reflected in all directions, and the particles, as has 

 been before remarked, becomes virtually a luminous point. 

 The result of these reflections at all the particles of the air is 

 the appearance of the bright blue sky ; and when we turn a 

 telescope towards any portion of the sky the field of view is 

 illuminated by what we may call the light of the sky, that is, 

 by t!ie light of the sun which has been reflected from the par- 

 ticles of the atmosphere. If, now, we direct the telescope, 

 during daylight, to a heavenly body, the field of view will be 

 illuminated gencrallij by the light of the sky ; and the part of 

 the field where the image of the body is formed will receive 

 the proper light of the body in addition to the light of the sky; 

 consequently, the visibility of the body will depend upon the 

 excess of light upon this spot, above the general illumination 

 of the field ; and it is obvious that this excess will bear a ratio 

 to the whole illumination, which will vary with the intensity 

 of the light of the sky. In order that the image may be visible, 

 it is found by experience that this excess must be equal to at 

 least one-sixtieth of the general illumination ; if this is not the 

 case, though the image of the body is reallj- formed upon the 

 field of view, our eyes are incapable of distinguishing it from 

 the surrounding parts of the field. Thus the greater the bright- 

 ness of the field produced by the atmospheric reflection, the 

 less will be our chance of seeing a luminous body situated be- 

 yond the atmosphere. As an illustration of this may be noticed 

 the power of seeing the stars by daylight through a long tele- 

 scope or tube. When the eye is unprotected, the retina is 

 illuminated by rays»reflected from nearly half the atmospheric 

 particles above the horizon ; whereas, when we look through a 

 long tube, it is only from particles situated in a comparatively 

 small region that the eye can receive light, and thus the light 

 of the star becomes of greater importance — bears a larger pro- 

 portion to the whole illumination — and may, if the tube be long 

 enough, become as much as one-sixtieth of the general illumi- 

 nation, and thus render the star visible. 



To return, then, to the moon's disc as seen beyond the sun 

 during the eclipse. Is it possible that the lumiere cendroe can 

 be the cause of its being thus visible ? If so, that light must 

 be equal to at least one-sixtieth of the light of the .sky received 

 on the field of the telescope in the neighbourhood. Now, this 

 twice reflected light is very feeble, and the exterior portion of 

 the moon was seen by Arago in 1842, when not more than half 

 the sun's diameter was eclipsed, and when the atmospheric 

 illumination must therefore have been far more than sixt}' times 

 the lumiere cendrce. Consequently this explanation of the 

 phenomenon is inadmissible. In fact, according to Arago's 

 explanation, which seems almost certainly the true one, the 

 moon's disc is visible under these circumstances, not because 

 the illumination of her image is greater than that of the sur- 

 rounding field, but because it is less so. It is seen just as the 

 portion of the moon which is between us and the sun, as a dark 

 object on a bright ground. This bright ground is an object of 

 considerable interest as proving almost conclusively the existence 

 of a non-luminous atmosphere of the sun surrounding the 

 luminous envelope, and capable of reflecting the light proceed- 



ing from the latter. The light reflected by this outer atmosphere 

 is so feeble t-hat under ordinary circumstances we cannot per- 

 ceive it, because it bears so small a proportion to the diffused 

 light of the sky : and even during the progress of an eclipse, 

 although the diffused light becomes more and more feeble as 

 the moon's shadow envelopes more and more of the atmosphere, 

 yet so long as any of the rays of the sun are not intercepted, 

 the light of the sky is strong enough to prevent our seeing that 

 reflected by the external envelope of the sun. AVhen, however, 

 the eclipse becomes total, all the direct light of the .«^un is cut 

 off from the portion of the atmosphere through which we are 

 looking, and the onl}- illumination of the sk}- in the neighbour- 

 hood of the sun is that produced by rays which have been 

 already reflected at distant parts of the earth's atmosphere. 

 The illumination thus produced is verj- feeble, and the light 

 reflected by the sun's atmosphere then becomes visible 

 as a broad ring of light or corona, surrounding the dark 

 body of the moon, and diminishing in brightness as it 

 recedes from the sun's disc; but the moment the sun re-ap- 

 pears, this corona vanishes again, so that there is no chance of 

 seeing it directly during any partial or annular eclipse however 

 large. When, however, about half the sun's disc is eclipsed, 

 the illumination of the field of a good telescope, arising from 

 the reflected light of the atmosphere, is considerably reduced, 

 and the excess of the illumination of the image of the corona 

 above that of the dark body of the moon becomes perceptible, 

 and renders the outline of the latter bod}' visible. The tnith 

 of this explanation is strongly confirmed \>y the fact that the 

 exterior portion of the moon is most distinctly seen near the 

 sun's limb; which is in accordance with the theory, inasmuch 

 as the corona, which forms the bright ground, is more strongly 

 illuminated in that neighbourhood than at a greater distance. 



We are now in a position to explain why this obser\-ation 

 would very probably fail, if the object glass of the telescope 

 were imperfectly polished, or had anj- particles of dust or mois- 

 ture adhering to it. If the object glass were perfectly transpa- 

 rent, every ray falling upon it (provided its path was not 

 inclined at too large an angle to the axis of the lens) would be 

 entirely refracted in its own proper direction. The illumination, 

 therefore, of the field of view, so far as it was due to the light 

 reflected by the atmosphere, would proceed from a compara- 

 tively small patch of the sky surrounding the point to which 

 the axis of the telescope was directed : raj's proceeding from 

 other parts would, after refraction, be so much inclined to the 

 axis as to strike the blackened sides of the tube of the teles- 

 cope, and so be absorbed and lost; so that such rays would 

 contribute nothing to the illumination of the field. Let us 

 suppose that with such a glass the dark exterior portion of the 

 moon's disc can just be distinguished; Ictus call the atmos- 

 pheric brightness 60; this will be the illumination of that part 

 of the field where the moon's image is,* the part immediately 

 surrounding this receives also the light of the corona, and since 

 it is percept ilj/y brighter than the moon's image, the number 

 expressing its illumination must be at least til. Now if we 

 suppose the object glass of the telescope to be imperfectly pol- 

 ished, or not perfectly clean, the rays coming from remote parts 

 of the sky will throw an additional light upnn the field of view; 

 for whenever one of such rays falls upon an opa(|ue spot or a 

 speck of dust on the object-glass a portion of it will be scattered 

 in all directions, making the spot virtually a new source of 



* Strictly spcnking. the moon's imngc would bo illuminated by tlio 

 lumifere ccndrfee, as well as by tho light of the sky ; but the former i« 

 so inconsiderable in amount, that it may beomitted. 



