Astronomy and Photography at Rome. 377 



So far may be considered as the first part of this most impor- 

 tant memoir. The sequel contains observations on the ring and 

 satellites of Saturn, and those by which the time of the rotation 

 of the planet Venus on its axis, has at length been determined, 

 and the spots of its disc correctly delineated. Ninety one designs, 

 on a small scale, of the appearance of the spots on the disc of 

 Venus, taken at various times, are annexed to the present memoir, 

 and a regular map, on a large scale, is announced for the next 

 publication. 



THE PLANET VENUS, ITS DISC, AND DIURNAL ROTATION. 



Alma Venus! coeli subter labentia signa 



Quae mare navigerum, qua? terras frugiferenteis 



Concelebras ; — quoniam — suaves tibi daedala tellus 



Summittit flores, tibi rident aequora ponti, 



Placatumque nitet diffuso lumine caelum, 



Adsis. Lucretius, lib. 1. 



From the time when astronomy became a regular science, the 

 construction of a correct table of the planetary movements, diur- 

 nal and annual, rotatory and orbitual, has been a main object of 

 research, and, in fact, the index of the progress made. The 

 knowledge obtained by the ancients, through a long course of 

 careful observation, received little or no increase, after the time 

 of Ptolemy, until Kepler arose ; for the Copernican system was 

 only a revival of the Pythagorean, the origin of which is lost in 

 the mist of time. But the discovery by Kepler of the law of 

 orbitual motion, accompanied as it was by the almost simultane- 

 ous invention of the telescope, gave a new impulse, and bestowed 

 precision and certainty on that which before was vague conjec- 

 ture. The times of the diurnal rotation of the planets Mars, Ju- 

 piter, and Saturn ; the position of their poles in space ; the incli- 

 nation of the planes of their equators to the planes of their sever- 

 al orbits ; the inclination of the planes of those orbits on the plane 

 of the ecliptic, and the longitude of their nodes, were soon deter- 

 mined with considerable accuracy. All this was done as far as 

 regarded those more distant, or what are called the superior plan- 

 ets ; but when the same points were sought for in respect to the 

 two inferior and nearer planets Venus and Mercury, new difficul- 

 ties occurred to baffle the best directed efforts. The abundance 

 of the light, illuminating Venus and Mercury, was found to be a 

 cause of greater obscurity than the scantiness of it at the distance 



Vol. xliv, No. 2.— Jan.-March, 1843. 48 



