OCCULTATION OF VENUS. 707 



Yenus occupies a conspicuous part of the field, there is no way to secure 

 the just optical effect, but by drawing Yenus with an apparent diameter far 

 greater tlian is due. Strictly, the lunar diameter should appear 73 times 

 greater than that of Yenus. Hence, if the apparent diameter of Yenus be 

 taken ,o inch, the apparent diameter of the Moon should be 7.3 inches. 

 Even the scale I have adopted in the diagrams scarcely gives the full opti- 

 cal effect, since the perfect semi-circle of Yenus, with its dazzling brightness, 

 was full advanced in the field, while only a small segment of the Moon 

 could be admitted. 



The unassisted eye could readily see, beside the bright crescent of the 

 Moon, the dark portion of her disc, rendered visible by the reflection of 

 light from the Earth. This feebly lighted surface, exhibiting what is called 

 the "ash}'- light," {la lumiere cendre) exists in Yenus just as in the Moon, 

 only it is barely distinguishable on very clear evenings, in powerful tele- 

 scopes. On the evening of December 8, Yenus was only three days from 

 her greatest eastern elongation, and hence her disc was almost perfectly 

 divided into semi-circles — one brilliantly lighted by the Sun's direct rays, 

 the other entirely dark — even the "ashy light" fading out in presence of 

 the Moon. For half an hour, to the unassisted eye, Yenus seemed to stand 

 still on the northeastern limb of the Moon, on the very border of "ashy 

 light." Some grew weary of watching it, and thus missed the moment of 

 immersion. At 5:20, (rlasgow mean time, the images of the Moon's dark 

 limb and of Yenus were brought into the field of our Clark Refractor — the 

 bright semi-circle of Yenus being turned towards the Moon's dark limb. 

 The limbs were then separated by an interval of three or four minutes of 

 arc. The purity of the air showed the Moon's dark limb to great advan- 

 tage — the sky beyond appearing intensely black in contrast with the "ashy 

 light." The border was most distinctly marked, and I patiently Avatched 

 the gradual approach of the bright and ashy discs, with finger on the break- 

 circuit key, so as to note on the Chromograph the moment. of contact. The 

 view was rare and beautiful. Even the brightest fixed star exhibits noreal 

 disc; but here two great spheres, projected as circles, were rapidly ap- 

 proaching each other. 



When the bright limb of Yenus was within eight or ten seconds of the 

 Moon's "ashy limb," a phenomenon occurred which has always been noted 

 in a superior degree in the transits of Yenus across the Sun's disc. It may 

 afford some indication of the extent and density of the atmosphere of Yenus. 

 instead of a sharp, closehj defined contact of discs, which was so fully prom- 

 ised, a border of wavering light, several seconds in vridth, seemed to pre- 

 cede the planet. This irradiation was such as to place the moment of ex- 

 ternal contact in doubt, by perhaps one second. The instant noted was 

 22A. 48m. 15.52s. Gl. Sid. time— corresponding to oA.^SGwi. 55.94.S. Gl. mean 

 time. This time must be taken rather as the instant of first appreciable en- 

 croachynent on the bright limb of the planet. The effect of this bright arc of 

 light was such, in transit observations of the British expeditions of 1874, 



