VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 5 1 1 



siderable mountains and elevated ridges; and indeed the most and the highest in 

 her southern hemisphere. This appears from the observations of the boundary 

 of illumination, which is not sharply terminated, and seems formed of light and 

 greyish shadow indistinctly intermingled. This is chiefly to be perceived only 

 about the time of the greatest elongation, when the eye looks perpendicularly 

 through the dense atmosphere of Venus, and by no means in the small crescent 

 form of light, when the lines of vision are much longer and more oblique 

 through that atmosphere: it is in the former position of the planet alone that it 

 can be seen distinctly, but even then not always equally so. One of the finest 

 scenes of this kind was afforded, for example, by the observation of the Qth, 

 when Dr. Chladni viewed the planet with me. A less striking inequality, though 

 perfectly certain, was discovered by my learned friend Dr. Olbers, July 31, 1793, 

 at 11*^ 5™ in the forenoon, which we both observed and delineated in the same 

 place, and exactly similar, after we had been observing since S*" IS'" in the 

 morning, but till that time saw no inequality. Were these small indentations or 

 darker places merely atmospherical, no reason can be perceived why they should 

 show themselves only in the boundary of illumination, and not in the other en- 

 lightened parts also. The same thing appears also from the irregular form which 

 the arch bounding the illumination sometimes assumes, and from the phenome- 

 non thence arising of the much smaller size of one horn, and particularly the 

 southern, in the crescent-shaped phases of the planet; as is shown, on the same 

 grounds, by the observations contained in my former memoir on the rotation. 

 Were these observations, as is alleged of the rest, nothing but fallacy, I should 

 wish to know the reason, why that deception happens only sometimes, continues 

 only some hours, and almost always takes place on the southern horn only, very 

 seldom on the northern. Whoever compares together the observations of this 

 kind contained in my memoir on the rotation, will find 14 in which the southern 

 horn appeared much smaller than the northern, but only 1 or 2 instances of the 

 opposite phenomenon. And, if it were merely deception, why does the smaller 

 horn, when the planet is seen through light clouds, always disappear sooner than 

 the broader one, and become visible again later? 



If any astronomer should think it worth the trouble to observe Venus, not 

 barely now and then, at whatever time of the day it may be, but continually, 

 with the same persevering zeal, and when the weather is favourable almost 

 hourly, about the time of her greatest distance from the sun, I am convinced 

 that he will certainly perceive the rare phenomenon in question, just as well as I 

 have done. If, contrary to all reasons which hitherto appear, I should hereafter 

 be convinced that I was deceived, I would myself, willingly and impartially, bring 

 the offering to truth; and so much the more readily, as no indirect views have 



