VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 235 



a pale greyish hue. After several other favourable observations Mr. S, infers 

 that, as there can now remain no doubt of the appearance of the pale ash-co- 

 loured streak of light, extending along the limb of the dark, hemisphere of 

 Venus; and as this planet cannot be said, like the moon, to receive some light 

 on its dark side from our earth, or any other heavenly body, it follows that this 

 light must either proceed immediately from the sun, which, as I have frequently 

 observed concerning the high mountains Leibnitz and Doerfel in the moon, 

 throws its rays directly on the tops of very lofty ridges of mountains; or else 

 that it is a light which partly illuminates the atmosphere of Venus, and partly, 

 being reflected by this atmosphere, marks out by a faint glimmer the limb of the 

 dark hemisphere of the planet, in the same manner as our morning and evening 

 twilight acts on ours. 



All our present observations militate against the supposition of this phenome- 

 non being the effect of light immediately proceeding from the sun; for, ]. This 

 light does not appear, as on the mountains Leibnitz and Doerfel in the moon, in 

 single, detached, and distant points; but as a continued streak of light, proceed- 

 ing from the extremities of the cusps, and continuing along the limb of the dark 

 hemisphere to a distance of about 8", or, in proportion to the apparent 

 diameter of the planet, no less than 15° IQ'of its circumference. This light 

 also, compared with the bright part of the phase, is not unlike the compara- 

 tively pale limb of the dark part of the moon before and after its conjunction. 

 2. Were this the light of the illuminated summits of a chain of mountains, 

 it would not appear so even, regularly connected, and spherical, as we behold 

 it. But what removes all manner of doubt is, 



3. The very difi:erent, extremely faint, bluish ash-coloured appearance of this 

 glimmering light, which forms as great a contrast with the whitish more vivid 

 light which is seen immediately on the cusps, as the ash-coloured light reflected 

 from our earth on the dark limb of the moon does, when compared with the 

 solar light on its phase. This pale light in the dark hemisphere, it must be 

 owned, faded away towards its termination, in the same manner as the solar 

 light did at the edge of the bright phase: but had this faint streak, like the 

 more vivid light, been an immediate emanation from the sun, the gradual dimi- 

 nution would have been throughout progressive in a continued proportion; and 

 the light in the dark part, immediately contiguous to the points of the cusps, 

 must have had nearly the same degree of brightness as the points themselves, 

 which was by no means the case. Every circumstance therefore seems to evince, 

 that this phenomenon is occasioned by a light reflected by the atmosphere of 

 Venus into the dark hemisphere of the planet, being in some measure the light 

 of the atmosphere itself, when illuminated by the rays of the sun, or, in fact, 

 a real twilight. But this will appear still more manifest when, 



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