September 15, 1892] 



NATURE 



469 



they were snow caps as suggested, perhaps they had been 

 previously observed. This was the case. On June 9 and 

 17, July 20, August I and 27, 1876, and February 5, 1877, 

 observations of these spots had been recorded in the note- 

 book, but owing to their not having attracted very great 

 attention at the time, they were regarded as ordinary 

 spots. That they are analogous to the white spots on 

 Mars is now undoubted : they have the form of a uniform 

 white segment of a circle, which, when seen edgeways, 

 appear as simple lines ; they are always exactly 180" 



February 



loh. 4sm. 



apart ; sometimes only one is seen because the other is 

 not lighted up by the sun : they are always approximately 

 near the terminator, and seem to oscillate backwards and 

 forwards, balanced, so to speak, around the axis of the 

 planet ; and, lastly, they are of a permanent nature, their 

 disappearances being due not to their annihilation, but 

 simply to the fact that they cannot be seen when receiv- 

 ing no light upon them. One main feature in which they 

 differ from the spots on Mars is that they neither increase 



Fig. 4. — The snow caps, February 25, 1891, loh. 15m. 



nor decrease with the seasons, at any rate to a sufficient 

 extent to be sensibly noticed. 



When Venus is in a favourable position for observation 

 many details on these spots have been recorded. 

 M. Trouvelot mentions here some bright spots (Figs. 5 

 and 6), which seem to be very numerous, and resemble 

 the bright specks which are seen on the terminator of the 

 moon, " sinon qu'elles sont plus brillantes, surtout sur 

 leur bord interne, et qu au lieu de petits crat6res, elles 

 sont entierement couvertes et hdrissdes de pics et 

 NO. TT94. VOL. 46] 



d aiguilles, qui, parfois, r^fl^chissent la lumi^re avec une si 

 grande mtensitd, que ce bord apparait tout constell^ 

 d dtoiles ahgndes comme les grains d'un collier de pierres 

 prL^cieuses, sans quelques irrdgularitcs dans cet aligne- 

 ment." The whole appearance seems to suggest that the 

 spots are at a higher level than the contiguous parts of 

 the planet situated at the edge. This idea is also further 

 borne out when the phase of the planet is a small cres- 

 cent, for then much more of the polar cap is found to be 

 visible than should be the case if the form of the phase 



, Fig. 5. — Details on the snov/ caps, January 19, 1878. 



was an exact crescent. In many cases a penumbra has 

 distinctly been seen, and in one of them it was so strong 

 and distinct on that part of the terminator lying between 

 the two polar caps, that it lasted for a month, the spots 

 remaining clear and brilliant throughout their entire 

 length. Ever since the year 1700, observers of Venus 

 have remarked these two spots that occupy the polar 

 regions. La Hire and Derham, observing the inequalities 

 of the surface at the extremities of the crescent, believed 



Fig. 6,— The snow caps, February s, 1878. 



that they could be produced by mountains higher than 

 those on the moon. Bianchini at Rome, Schroeter, 

 Gruithuisen, and several others, all have reported the 

 existence of such markings, but they were never led to 

 conclude that they were snow caps analogous to those 

 on Mars. 



To obtain a general idea of the rugged ness and smooth- 

 ness of the planet's surface, the terminator has helped to 

 considerably distinguish the high and low elevations and 

 depressions respectively. The surface of Venus from 



