434 NEW OBSERVATIONS OF THE PLANET MERCURY. 
many dilTerent eyes. At times a small aperture is better than a large one, at other 
times not, the relative efficiency of the glass depending upon the condition of the air. 
In Mexico I was able to test this very effectively by having the 6-inch clamped on 
to the 24-inch tube, so that it shared the advantages of the other's mechanism, and, 
what is as vital in such a test, it enabled the two images to be compared almost at 
the same instant by the same eye. I found that the performance of the 6-inch was 
arly equal to that of the 24 
revealed almost as man} 
markings as the larger glass; and that when the two differed in efficiency it was 
apparently question of the kind of air-waves prevailing at the moment. In the 
moments of best seeing, the 24-inch of course distanced its competitor; but the 
detection of the principal markings does not demand superiority of the sort. 
Secondl//. Between fairly normal eyes there is practically no such thing as superior 
keenness of vision." After a little practice in the subject one eye proved as good as 
another. Not keenness of sight, but keenness of brain, distinguishes the good from 
the poor observer. It is with astronomical observation exactly as it is with every- 
day affairs. The able observer is he who perceives what any one may see. 
Due neither to aperture nor eye, we come now to what it was due, to wit, to the 
air of the Observatory site, backed by persistence in the observer : (1) the air must be as 
steady 1 — not as clear — as possible; (2) the observer must study his subject. To 
see the markings it is necessary, for an unpractised eye, that the air should be steady 
enough to show the disk clear-cut. Althouo-h I have been able to detect the mark- 
ably in air such that I could not see 
of the disk, such 
would not suffice to show them to one not versed in the subject. The above criterion 
IS sufficiently exact to begin on ; later, the observer will discover that sharpness of 
limb and appearance of detail depend upon special and different atmospheric condi- 
tions, and do not always 1^0 too-ether 
With regard to the second point, the best results are got by looking for the planet 
at times not usually thought practicable. First, the planet must be scanned by day 
exclusively, and as near high noon as possible. Secondly, the planet can and should 
be followed almost up to superior conjunction. The best time to study him is when 
planetary almanacs state, "Mercury invisible during the month." 
^^ 111 the next place, although the size of the object-glass used is largely immaterial, 
' - power of the eyepiece is not. High powers will show nothincr. I had eyepieces 
pecially made for me ; even the 
d on large telescopes p 
to llZ '-^T '°"'*^\^^^'' steadiness I refer the reader to a preliminary monograph by Mr. Douglass 
to .horn .s due the detection and study of its cause, in Popular Astronomy for June, 1897. 
