HOW TO SKETCH THE MOON. 
237 
we shall find it far more easy to draw with fidelity an obliquely 
■viewed landscape, in which circular forms prevail, as well as 
to estimate correctly the amount and direction of such devia- 
tions from perfect circularity, as Avill probably be found. But 
(2) supposing correctness of form to be ensured, we have 
another point of equal or sometimes even greater importance 
to attend to, the relative proportion of size. The neglect of 
this has, as much as anything, retarded the solution of the 
interesting question whether volcanic activity is still in pro- 
gress on the moon. The minute craters which abound in so 
many lunar landscapes have been often jotted down, as if the 
observer was quite satisfied with recording their existence, and 
attached little importance to their relative dimensions ; yet 
this is the only criterion, as it is an equally easy and satis- 
factory one, of their activity or extinction. And (3) the rela- 
tive position of objects should also be attended to. Less 
material than the two preceding, it must not be neglected, as 
an element in the required faithfulness of representation, and 
in some cases as avoiding considera-ble trouble in identification. 
And we may add here a caution against finishing off a careful 
•design with a sketchy margin of general resemblance, which 
at some future time, when the limits of accuracy are no longer 
remembered, may prove a source of perplexity and annoyance. 
There is some latitude as to the materials which may be 
employed. Those who can handle a pencil dexterously may 
find it answer well; but they should not forget to set the 
sketch afterwards. Delicate pen and ink drawing, in the 
manner of etching, may be made very effective, but is not easy 
of correction in case of mistake. With any one accustomed 
to the brush, watercolour lamp-black succeeds very well — not 
Indian ink, as it is troublesome to get it black enough for the 
deep shadows, and is too indelible in case of error. Professor 
Piazzi Smyth used oil colour ; but this is less easily managed, 
excepting by a practised hand. Black and white chalk on a 
grey surface gives a striking effect, but independently of its 
great liability to damage, it requires an inconveniently large 
scale, or with these materials the requisite minuteness will be 
unattainable. Probably the most unexceptionable mode (I regret 
that I cannot speak of it from personal experience) would be 
the employment of tube-colours, consisting of black and white, 
and admitting of ready mixture, and correction where requisite, 
on a ground of neutral grey composed of the same materials. 
In this way there would be little difficulty in representing 
those ‘ half-tones,’ or gradations of shade, the want of which 
is so apparent in ordinary lunar drawings to an experienced 
eye. The most essential purposes may no doubt be secured 
without them, and their introduction requires a good deal of 
