BY A. B. BIGGS. 47 



annulus round Mercury and the White Spot on Mercury 

 during transits across the sun may now be regarded as 

 regular concomitants of the phenomenon ; but there is no 

 agreement amongst astronomers as to the cause of these 

 appearances. The White Spot has been regarded by some as 

 indicative of volcanic action, but this seems mere fancy. 

 Prof. Powell, with more show of reason, suggested that 

 diffraction of light had something to do with the matter. 

 . . . . Huggins rejects this theory. It might conceiv- 

 ably have its origin in the internal reflection of light in a 

 Huygenian eye-piece." (I was using a Ramsden or positive 

 eye-piece). Webbe, in his "Celestial Objects," says that it 

 " arises probably from deception from the violent contrast 

 and the fatigue of the eye. . . . No terrestrial analogy 

 will explain a luminosity thus visible close to the splendour 

 of the Sun, and it seems natural to refer it to the exhausted 

 state of the retina " (The haze on this occasion precluded 

 the operation both of " violent contrast and fatigue.") A 

 possible explanation may, perhaps, be found in connection 

 with the atmosphere of Mercury. The " annulus, " which is 

 usually seen around Mercury during transits, would seem to 

 indicate the existence of an atmosphere of considerable ex- 

 tent. (I failed to see it on this occasion, though, I believe, 

 -so far as I can recollect, that I saw it during a former 

 transit ; but unfortunately I took no notes at that time. The 

 haze, however, would sufficiently account lor my not seeing 

 it this time.) This body of atmosphere would act somewhat 

 as a lens (or rather as a " spot lens ") and would form a 

 diffuse focus or image of the sun on this side the planet. 

 We should not under ordinary circumstances see this image, 

 because the rays would cross and pass by on each side of us, 

 unless there were some sort of translucent screen on which to 

 receive it. Such a screen might possibly consist of the 

 meteoric matter which is supposed, or I may say is known 

 to exist in interplanetary space, some of which reaches us in 

 meteoric showers, and some probably mere planetary dust, 

 occasioning the phenomenon known as the " Zodiacal Light." 

 (Of course every particle of this would be moving in some 

 regular orbit, in accordance with gravitation law.) This 

 may be deemed a very fanciful theory, and possibly may 

 excite ridicule from mature scientists ; but slow progress will 

 be made in scientific knowledge if it is to be impeded by such 

 a fear. I have not overlooked the fact that the outer or upper 

 stratum of Mercury's supposed atmosphere would be more 

 rarefied and less refractive than the lower ; in fact it would 

 be unequally refractive throughout; but this, I think, would 

 be rather in favour of the theory, inasmuch as it would 

 lengthen the focus and bring it nearer to us. 



