110 TEA2fSIT OF TEXUS. 



it is one of those curious phenomena seen only by some observers 

 under special conditions. During the transit of Mercury in 1868 

 it was watched very closely by a number of observers in England, 

 who were seeking information that might be useful for the transit 

 of Yenus ; and out of fourteen observers, including some of the 

 best in England, only three make any mention of exterior halo. 

 Mr. Stone thought it simply an effect of contrast. 



Probably some of the light seen on the planet this time had a 

 similar origin, for no observer has, so far, reported seeing both. 

 A part of it, however, must, I think, be attributed to haze in our 

 own atmosphere, which, being very luminous owing to moisture 

 then forming, would appear projected on the black planet, and 

 the contrast would very likely give it a shaded appearance from 

 the edge towards the centre. To me the blackness of the planet, 

 both at ingress and egress, was very intense, until the haze in our 

 atmosphere became thick and gave the surface of the planet a 

 cloudy look, so that I could scarcely see where the planet ended 

 and the slcy began ; and it may be that the same cause produced 

 what Professor Liversidge and others saw ; but at Woodford the 

 air was too clear for such an explanation. The red tint seen by 

 Professor Liversidge is explained by his having used a red glass 

 shade. 



The increase or thickening of the halo seen at the north pole 

 of the planet, and which to several of the observers seemed to 

 encroach on the planet, is a most interesting featui'C. 



The remaining ring of light or halo is, however, the most inter- 

 esting physical feature observed, though at first sight it would be 

 attributed to an atmosphere similar to that of the earth. I think 

 a little consideration will show that it cannot have such an origin. 

 It is spoken of by all the observers as very brilliant, by some as 

 white compared with the sun ; and its actinic power was so great 

 that, although its diameter was certainly less than one second of 

 arc, and would only appear as a fine line in the photo-heliograph 

 less than one five-hundredth part of an inch in diameter, it yet 

 had power to aftect the chemicals in something less than the two 

 hundred and fiftieth part of a second ; in other words it was 

 quite as powerful, if not more so, than direct sunlight, and we have 

 a number of photographs in which it appears. 



This great brilliance, of course, explains why it was not seen 

 about the planet while on the sun's disc. It was evidently not 

 to be distinguished from the sunlight. In the clear atmosphere 

 at AVoodford it was seen as soon as the cusp parted at egress, 

 and it will be exceedingly interesting next time Venus is lost in 

 the sunlight to try if, under favourable conditions, the halo can 

 be seen. Quite sure I am that, if the air had been clearer at 

 egress, I should have seen the planet with the halo round it 

 projected upon the sky ; as it was I saw part of the halo until 



