236 



KNOWLEDGE. 



[October 1, 1897. 



it, for it was by the white spot that he thought he could 

 determine the movement of the planet. But he never saw 

 the white spot again, nor has anyone else either. It may, 

 of course, have been real, and been caused by a mass of 

 clouds carried by an aerial current in a northward direction ; 

 but it is more closely connected with the terminator than 

 with the planet itself, and must be au optical eilect. 



This determination of the rotation is, therefore, of no 

 intrinsic value. Be this as it may, the rapid displacement 

 would point to a period of less than one day, as stated by 

 Cassini, instead of twenty-three days, as has been said. In 

 his work, Astnmomie, Jacques Cassini, wishing to reconcile 

 the observations of Bianchini — of which we shall speak later 

 — with his father's, concluded that the period is probably 

 twenty-three hours and twenty or twenty-two minutes. 



(2.) In 1726 and 1727, Bianchini made a series of 

 observations, both at Rome and Albano, with the aid of 

 an excellent object-glass constructed by Campani. " These 

 observations and these drawings we have now before us. 

 In no particular do they resemble or tally with those made by 

 Cassini. Bianchini always saw circular dark spots, cut by the 

 terminator. We here reproduce the two most characteristic 

 sketches. There will be noticed three spots which have 

 shifted in the downward, or southward, direction. The obser- 

 vations were made in the evening, at sunset, and the author 

 takes care to inform us that he has represented the phase in 

 situ vero, and not inverted. He estimates the displacement 

 of the spots at fifteen degrees a day, and concludes a rota- 

 tion of twenty-four days and eight hours, the equatorial 

 line being inclined seventy-five degrees to the ecliptic. 



These spots are just as uncertain as those of Cassini's. 

 The luminosity of the crescent of Venus is always much 

 enfeebled as it gets nearer to the terminator, as it is there 



that the mini- 

 "N^_ mum of light is 

 received from the 

 sun. All that is 

 noticeable is a 

 somewhat vague 

 variation of 

 shade, and often 

 in Bianchini's 

 o b se rva tions 

 themselves one 

 and the same 

 observation i s 

 illustrated by two 

 drawings, between which there is scarcely any resemblance 

 — nay, some of which very appreciably differ. Thus, for 

 example, on January 7th, 1728, there are shown four large 

 spots on one drawing, and only three on another. Also, all 

 these patches are shown to be of a circular shape. Now, 

 although the Italian astronomer considered these patches as 

 sufficiently certain andpermanent togivethemnames (Chris- 

 topher Columbus's Sea, Vespucci's Sea, Galileo's Sea, etc.) 

 and to construct a map or chart, and a globe too, nothing 

 can be less certain than their existence or situation. 



(3.) A third observer, after Cassini and Bianchini, to 

 study the planet Venus, and who did so in a particularly 

 painstaking manner, was Schroeter, the able and industrious 

 astronomer of Lilienthal. He commenced his observations 

 in 1779, and continued them until 17!'5. His observations 

 were published by him in a remarkable work which we also 

 have before us.t It is illustrated by a very large number 

 of drawings, not one of which shows any spots bearicg the 

 least resemblance to those of Cassini or of Bianchini. An 



* Sesperi et Phoxphori nova Phenomena. (Kome, 1728.) 

 + Aphrndilnqraphixchf Frrtfimenfe. (nolmstcflt, 170!i.) 



Fl». 4. — A Sketch of Venus by Schroeter, 1788. 

 (L'Astronomie, 3890, p. 331.) 



Fiff. 



— A Sketch of Venus by William 

 Herschel, 19th June, 1780. 



idea of Schroeter's may be formed by the inspection of 

 Fig. 4. 



This observer's first impression was that those spots or 

 marks were probably nothing but illusions. And, in truth, 

 that faint, dark image, remaining motionless for two 



months, can 

 be nothing but 

 either an opti- 

 cal illusion 

 or an atmo- 

 spheric efi'ect. 

 This, too, is 

 what Schroe- 

 ter himself 

 says. In our 

 terrestrial at- 

 mosphere we 

 often find that 

 certain as- 

 pects, such as condensation, a temporary clearing ofl' 

 of mists, and the like, reappear at the same hours for 

 several days in succession, being dependent upon the sun. 

 However, the Lilienthal astronomer, having noticed a 

 little later on, in 1792 and 1793, certain uneveunesses or 

 jagged edges, of a rapidly varying form, especially at the 

 cusps of the crescent, concluded that the time of rotation 

 was twenty-eight hours twenty-one minutes. That was 

 also the period of time assigned to it by Jacques Cassini, 

 from his father's observations. Schroeter was influenced 

 by those figures, for it is easy enough to see that it was 

 absolutely impossible to deduce the time of rotation from 

 the mere reappearance of a few more or less uncertam 

 white points upon the terminator of the crescent of Venus. 

 (4.) The fourth observer worthy of notice who has dealt 

 with the physical aspects of Venus is the immortal Sir 

 William Herschel. He, however, expresses little satisfaction 

 at the results he achieved in this respect, although he had 

 started his observations with the conviction that Cassini and 

 Bianchini had seen something real. He appears to have 

 published his paper'- for the sole purpose of criticizing the 

 confidence with which Schroeter announced that he had 

 discovered mountains upon Venus six times higher than 

 Chimborazo, ('.'., about twenty miles high, and that he had 

 ascertained its rotation. Herschel denied emphatically the 

 existence of any mountains on Venus. He does describe 

 certain faint spots which he believed he had seen upon a 

 few occasions, and whilst crediting the idea of the planet's 

 rotation, he declares that neither the sense nor the period 

 of the rotation is to be deduced from them. From this 

 memoir, which I have also consulted, I extract Fig. 5, 

 drawn by himself on the 19th June, 1780. In his opinion 

 these spots frequently assume the aspect of optical illusions, 

 or, as he expresses it, " optical deceptions." 



For an astronomer like Sir William Herschel to give up 

 the observation of Venus, means that he must have recog- 

 nized the impossibility of discovering anything from it. 

 {To lie colli ill Hid.) 



3Lcttcrs. 



— * — 



[The Editors do not hold themselves responsible for the opinions or 

 statements of correspondents.] 



E SCDTI AND OTHER VARIABLES. 

 To the Editors of Knowledge. 



Sirs,— From the apparent maximum of R Scuti on 

 February 27th, noted in Knowledge for June, until the 



* "Observations of Mie P];inct Venus." Koval Societv, 13t]) -Tune 



