Page:Popular Science Monthly Volume 75.djvu/527

Rh also came out, developing into a sort of collar round the southern pole. Spots, too, small, not large, stood blotched upon the disk. In this investigation not only was care taken to guard against illusion, but no regard at the time was paid to any previous observations.

The configurations thus disclosed proved permanent in place. By watching them assiduously it was possible to note that no change in position occurred in them upon the face the planet showed, first through an interval of five hours, then through one of days, then of weeks. It thus became evident that they bore always the same relation to the illuminated portion of the disk. This illuminated part, then, never changed. In other words, the planet turned always the same face to the sun. The fact lay beyond a doubt, though of course not beyond a doubter. The fundamental importance of this primary fact upon the world of Venus we shall note as we proceed.

In character these markings were peculiar and distinctive. In addition to some of more ordinary form were a set of spoke-like dark rays which started from the planet's periphery and ran inwards to a point not very distant from the center. The spokes began well-defined and broad at the edge, dwindling and growing fainter as they proceeded, requiring the best of definition for their following to their central hub. They were most noticeable on the edge of the disk which marked the boundary of light and shade, the sunrise line the terminator, as it is called; less so on that which the sky cut off, called the limb.

From so much of the planet as was then presented earthwards, it was possible to make a map giving the chief features of Venus's globe. In addition to demonstrating the durational identity of the axial rotation and the orbital revolution, the markings showed the planet's poles to be practically perpendicular to the orbital plane. Thus the central equatorial longitude lay in perpetuity directly under the sun.

The difficulty in seeing these tell-tale marks lies in their faintness. A very good reason for such astronomic concealment on the part of Venus will appear as we go on. In consequence, the contrast must be intensified as much as possible and to secure it a small aperture and low powers are best. It is only in very good air that these helps to definition can be disregarded.

Corroboration of these markings has been obtained at Flagstaff in the years that have since elapsed. In 1903 a bulletin giving drawings of that year was published, showing confirmation, and in 1907 and 1909 other drawings afforded like witness to their actuality. Markings have been seen by almost every member of the staff and independent observations made on identical dates show remarkable agreement.

But the most telling testimony is the concordance in results between different methods of investigation. The first of these we shall mention is the spectroscope.

The spectroscope is primarily an instrument for analyzing light.