. ON THE VISIBILITY OF THE DARK SIDE OF VENUS. 407 



stood the word in its modera sense, meaning substances which absorb 

 sunlight and emit it in darkness without being chemically changed, or 

 whether they included under that name, like all the elder physicists, slow 

 combustions also, like that of phosjihorus and rotten Avood, which in modern 

 terminology do not belong to true phosphorescence. In both cases it is 

 difficult to imagine the whole surface of the planet to be covered with 

 such substances as sulphide of strontium, diamond, phosphorus, or rotten 

 wood. 



2. Auroral phenomena. — This was partly Schriiter's idea; it is supported 

 by a most extraordinary observation of ITadlcr, who, during the whole 

 evening of April 7, 1833, saw Venus surrounded by long bright immovable 

 rays. Professor ZciUner, of Leipzic, strongly advocates this idea, and trusts 

 that the spectroscope will reveal bright lines in the grey light of the unillu- 

 mined hemisphere of Venus. 



3. Proper light. — An explication upheld by Pastorff, who supposed the 

 atmosphere of the planet to be large and self-luminous. Possibly also the 

 planet might still be incandescent, as is supposed to be the case of Jupiter by 

 Mr. jS'asmyth; but on this supposition the secondary light should be always 

 visible, which is positively not the case. 



4. The light of the Earth. — This, as seen from Venus, far exceeds the 

 greatest brightness of Venus as seen by us; and according to the calculation 

 of Dr. Pthuinaucr, of ilunich (Grundziige der Photometrie, 1861, pp. 58-77), 

 the grey light of Venus, if resulting from this cause, should equal a star of 

 the 14th magnitude. That this explanation is insufficient is so clear as to 

 need no further proof. 



5. Negative visibiliti/, as it is called by Arago, or projection on the coronal 

 light of the sun, as suggests Mr. Lynn (Astr. Reg. No. 109, p. 12) and, if 

 I am right, Mr. Noble (Month. Not. vol. xxxii. p. 17). This explanation 

 suits only those cases in which the unillumiued side of the planet was seen 

 darl-er thau the surrounding sky (Messrs. Browning and Noble), but not 

 those of the majority of observers, who make it brighter than the sky. 



6. Accidental combustion and other illumining processes. — Gruithuisen 

 suggests large luxuriant forests set on fire, an idea by no means absurd iu 

 itself; but, indulging in the fantastic cast of his mind, he brings it in 

 connexion with general religious festivals of the inhabitants of Venus, a 

 speculation in which it is not quite easy to follow the famous Munich seleno- 

 grapher. Immense prairies and jungles would do still better ; but even 

 these will hardly suffice for so frequent and general a phenomenon. 



I wUl suggest another explanation, without laj-ing too much stress on it, 

 though perhaps it is not a mere fancy. The intense brightness of Venus, 

 and particularly the dazzling splendour of her bright limb, is deemed by the 

 late G. P. Bond and by Professor ZoUner, a competent authority in photo- 

 metric matters, not to be explicable without assuming specular reflection on 

 the surface of the planet. This Professor ZoUner supposes to be done by a 

 general covering of water ; and iudeed if the faint grey spots of Venus, 

 delineated in 1726 by Bianchiui and rediscovered by Vico in 1838, are land, 

 then nine tenths at least of the surface of Venus are covered by sea. Should 

 Venus be in a geologically less advanced state, viz. less cooled than our 

 globe, a supposition rendered not improbable by her considerable size and 

 her nearness to the sun, then the present condition of Venus would be 

 analogous to that of the earth in the Jurassic period, when large isolated 

 islands were bathed by immense seas, blood-warm, and teeming with an 

 abundance of animal life difficult to be conceived. 



