6 REPORT—1868. 
class for those of the second. Posidonius y has not been observed so assidu- 
ously as Linné; it has, however, been found to retain the appearance of a 
mountain to a later period in the lunar day, when Limné has exhibited nothing 
but the ill-defined nebulous spot. 
Iv A*17 TV A¢#9, which differs from Linné and Posidonius y, is a crater 
opened in the bottom of a depression which occurs in a mountain-chain on 
the east border of Hipparchus, Similarly to Linné and Posidonius y, it is dis- 
tinct and well defined shortly after sunrise, but, unlike them, it retains this 
appearance for a much longer period of the lunar day. Under a low solar 
altitude the eastern interior side of the depression in which the crater is situ- 
ated is very bright, showing that for some time after sunrise it must be in- 
cluded in the first class of bright spots. On some occasions, when the sun 
has attained an altitude of about 20° above its horizon, it has presented a 
very similar appearance to that of Zinné and Posidonius y. On other occa- 
sions this appearance has not been observed until the sun has attained an 
altitude of nearly 40°. A bright streak has also been noticed extending from 
it towards the east, while the crater-form has been distinctly visible, with a 
solar altitude of nearly 30°. With solar altitudes above 48° to meridian 
altitude 85°, the appearance has been that of two nebulous bright spots of the 
second class. 
While the three objects under high solar altitudes present precisely the 
same appearance, the lunar surface in each case differs materially :—Linné, a 
small isolated cone with crater opening (?) on a comparatively level surface ; 
Posidonius y, the highest point of a mountain-ridge, having a minute perfo- 
ration ; TV A*17 TV A$ 39, a somewhat large opening, with a small central 
one, in a depression also upon the summit of a mountain-range. Itis clearly 
the province of observation to endeavour to ascertain in each case the circum- 
stances which are intimately connected with the jist appearance of the white 
spot. Apparently these spots appear to be on the surface. In the case of 
Linné the spot spreads around the cone or crater; in that of Posidonius y it 
extends around the summit of the mountain, while in that of [VY A*!7 
IV A¢ *9 it covers the depression and included crater. Of the four conditions 
of brightness in the first class of bright spots, the sun’s altitude appears to be 
connected with the first appearance of the white spot in the second class, but 
not regularly so, inasmuch as it appears earliest in Linné and latest in 
IV A*17 TV A¢#9, The discussion of observations is not sufficiently advanced 
to ascertain if the sun’s azimuth at all affects these spots. The third and 
fourth conditions of the first class are clearly inapplicable to spots of the second 
class. 
It has been suggested that the nature of the surface in these localities is 
such as to reflect the hazy light we see. We have in the three examples 
before us three different kinds of surfaces, as above mentioned. The greatest 
similarity of appearance of the white spot occurs in those cases in which the 
surfaces are most dissimilar, viz. a nearly level plain from which rises a 
small cone, and a mountain peak having a small orifice. While the surfaces 
are dissimilar, the feature in which the two objects closely resemble each 
other is the minute orifice which has been seen in both. We have conse- 
quently in two objects greatly differing from each other, two very close points 
of resemblance—the minute orifice seen with very low solar altitudes, and the 
comparatively large white spot seen under higher altitudes. 
al 
