The Past and Coming Traiisits and Arctic Explorations. 311 



ately after the fii-st identation at the sun's limb had been observed, M-. 

 Mouchez began to measure the distance between the cusps. " About 

 a quarter of an hour," he proceeds, " after the first contract, when half 

 the planet was still outside the sun, I perceived suddenly the entire 

 disc of Venus, defined by a pale halo, brighter near the sun than at 

 the summit of the planet" (that is, at the part remotest from the sun's 

 edtre"). "To make sure that I was not under an illusion as regards 

 this unexpected phenomenon 1 immediately reversed the position- 

 circle of the micrometer to 180°, and measured that diameter of Venus 

 which was still partly outside the sun, and I found it identical with the 

 diameter perpendicular to the lines joining the centres ; it was, there- 

 fore, really the entire well-defined disc of the planet which I saw. 

 But in proportion as the second contact approached, the two extreme 

 portions of the halo nearest the sun (and more distinctly seen) tended 

 to unite, surrounding the segment still exterior to the sun with a 

 brighter light, and this too early union {reunion anticip6e) of the cusps 

 by a lummous arc was rendered still more perfect by a narrow border 

 of very bright light bounding the aureole on the disc of Venus. 

 Foreseeing at once that there would be great difficulty in observing 

 the geometrical contact, even if it would not be absolutely impossible, 

 I quickly changed the darkening glass of pale blue for one of deeper 

 tint, by means of which I hoped to extinguish this halo with its 

 accidental gleams (Imurs accidentelles) , but it was useless; the halo 

 still remaining visible, I was obliged to take the original darkening 

 glass again. Under such circumstances, I had to take the moment ol 

 contact, not the meeting of the two cusps, or the geometrical contact, 

 but the moment when the sun's disc no longer seemed disturbed by the 

 bright light which enveloped the planet at the point of contact. I 

 observed a very sensible time-difference betAveen the moments when 

 I believed the contact might have been established and the moment 



when 1 felt absolutely certain that contact was established 



The third contact was also observed under excellent conditions, in a 

 very clear sky between clouds, with the same phenomena as at the 

 second contact, but in reverse order." 



It needs no elaborate argument to show that this peculiarity must 

 altogether prevent the observation of contacts with the degree of 

 accuracy necessary to improve our estimate of the sun's distance. 

 With the improvement of telescopes the phenomenon will only be so 

 much the more clearly recognized ; and it must be quite impossible to 

 experiment on this phenomenon as on the "black-drop," seeing that 

 we can not represent by a model the action of an atmosphere whose 

 real extent and refractive power are unknown to us. 



