244 
POPULAR SCIENCE REVIEW. 
The determination of the rotation period, or rather, obser- 
vation for correcting the rotation period, needs no special 
•explanation. We have already seen how the time when the 
various features of the planet’s surface will come to the central 
meridian can be determined beforehand. It is only necessary 
to note the actual time when they do come to the central 
meridian, to ascertain whether any correction is required. But 
in point of fact, observations made on the planet’s rotation now, 
will only have any real value some century or two hence. The 
rotation period has been already calculated to within the tenth 
part of a second, for though Kaiser’s result differs from mine bv 
about that amount, I have shown that there are clerical errors 
in his calculation, (such, for example, as his taking the years 
1700 and 1800 as leap-years,) and that when these are corrected 
the same rotation period within a 5 0th part of a second results 
from his researches as from my own, 24 h . 37 m . 22*7 S . 
It remains only that I should consider what special ob- 
servations of the features of Mars are now likely to be of ser- 
vice. 
In the first place, I think the time has come for a more 
careful study of the varieties of light and shade and of colour in 
this interesting planet. It should be noticed that the apparent 
discrepancies between many excellent drawings are probably 
in the main due to this cause. I have studied hundreds of views 
of the planets, and at first 1 used to be greatly perplexed by 
finding that two skilful observers seem to see two different 
planets with their telescopes. The drawings constructed by one 
observer agree most satisfactorily inter se, and so do those 
obtained by the other; but when one set is compared with the 
other the most startling discrepancies are noted. I am disposed 
now to attribute this chiefly to the fact that slight varieties of 
shade have not been sufficiently noted, or, if so, have not been 
adequately indicated. In the main, observers are apt to divide 
the surface of Mars into two tints, one light, the other dark, and 
one observer will set a portion which is faintly shaded in the 
dark part of his picture, while the other not recognising the 
difference of shading, perhaps, or else considering it unimpor- 
tant, sets that portion in the light part. As the colouring of 
Mars is in reality exceedingly delicate, especially in certain 
portions of the planet, and as, moreover, different eyes differ 
greatly in their estimate of colour, the drawings are not cor- 
rected on this account, as otherwise we should expect. The 
•ordinary text-book notion that the surface of Mars is divided 
into a ruddy portion, a green portion, and the white polar 
snow-caps — with perhaps occasional white cloud-markings — is 
altogether remote from the truth. Only a very small portion of 
the land has a ruddy tint which can be regarded as well defined, 
