516 REPORT—1868. 
(3) Also, without either clouds or fog being distinctly observed, the trans- 
parency of our atmosphere is very different according to time and place, and 
little faint projecting forms may easily become alternately visible or invisible 
without adducing any other cause than the changeable diaphanism of our 
atmosphere. This alternation is most conspicuous in the so-called “ rillen” 
(furrows), which, with few exceptions, are scarcely visible. I could point out 
many places which under the most favourable circumstance, of the air, appear 
streaked by numerous little rills, while at another time nothing, or nothing 
notable of them, is to be seen. An exception must be made only in the case 
of those which are so wide that we can perceive their shadows, as the rills 
near Huyghens and Aristarchus. Schmidt at Athens, who published a pam- 
phlet exclusively on the rills of the moon’s surface, has made the same remark. 
Again the green colour of some of the moon’s landscapes is scarcely visible ; it 
generally appears only when the moon is either quite or nearly full, therefore 
all the shadows are wanting. At that time, under favourable circumstances, 
the greatest part of the Mare Serenitatis appears of a uniform green colour, 
with a defined edge towards its blackish-grey margins. At other times these 
edges are observed, which are, however, but a lighter grey separated by a 
darker; but it is more difficult to distinguish clearly this green in Mare Crisium 
and Mare Humorum from the grey. 
Hitherto telescopes of large dimensions have not been employed, or at least 
not continuously, for the moon, for which we can find sufficient reason. If 
gigantic instruments are to manifest their full power on these objects, not 
only occasionally, but in an unbroken succession of nights, it is a matter of 
necessity that they be not placed in Northern or Central Europe, and espe- 
cially not in the lower regions of the atmosphere. Also they must admit of . 
a lighter and speedier manipulation, that it may be possible to direct them 
to all parts of the celestial vault. For double stars and nebulous spots, it 
may suffice that the tube is movable only in or near the meridian ; but this 
is by no means a sufficient condition for the moon, and we have never expected 
that Lord Rosse’s telescope would advance our moon knowledge. It is not 
here exclusively the question, in the first instance, of the utmost magnifying 
power; but quite different, and more difficult conditions must be realized. 
We have learned, through the meritorious labours of Piazzi Smith, the unex- 
pectedly great superiority which the Peak of Teneriffe offers to astronomical 
observations ; and we can adduce the experiences of others, which unite in 
proving that the highest possible stations in a tropical, or at least subtropical 
country are to be fixed upon as the most fit for observations of extreme deli- 
cacy. In Italy, Dominique Cassini had seen spots on the disk of Venus ; in 
Paris, though employing a more powerful instrument, he failed. I myself in 
Dorpat, and Lamont in Munich, have sought in vain for these spots which De 
Vico and Secchi found in Rome, and made use of for the determination of 
that planet’s rotation. In South America, Humboldt could see stars in the 
Great Bear which in Europe he in vain sought, although the constellation has 
a much greater altitude. Neither would Lassell have changed his residence 
to Malta, had he not been aware of the great difference between the two 
climates: where the shadows of the trees can plainly be perceived by Venus’s 
light, there also more can be attained for the moon than in our northern 
climate. 
But in order to observe our satellite uninterruptedly and successfully, it 
must be possible to turn our instrument to every part of the heavens. We 
must be able to observe the first visible sickle in the W. or NW. after the 
new moon, and in the same manner the last in the E. A telescope which 
