may often be made instructive in observing solar phenomena. By closely examining it, it is sometimes possible to detect the presence of spots on the solar surface. Solar eclipses may be watched in the same way. As the moon gradually encroaches upon the sun's disk its progress can be traced by a corresponding change in the form of the image, which resembles that of the uneclipsed portion of the solar surface. In such observations, however, it will be remembered that the course of the movement is always reversed. It was in this way that the transit of Venus was first observed by Jeremiah Horrocks, November 24, 1639. He had calculated the time, which fell upon Sunday morning. He arranged his room for the observation, and then went to church, as he did not wish any secular interests to interfere with religious devotion. It is very probable, however, that Venus was mixed up with his devout meditations, for he hurried back from service, and was delighted to find that his calculations were verified, as the planet was already far advanced in its passage across the sun's face.
We are all familiar with similar images of the sun, of a round or oval outline, formed upon the ground where his rays shine through small openings in the foliage of trees, as illustrated in Fig. 5.
Fig. 6.
Formation of a Shadow.
The same property, that is, the rectilinear propagation of light, gives rise to a reverse effect, or a sort of dark image or shadow, although a shadow depending upon the absence of light is of course not properly an image. "Let the source of light be a luminous point, and let an opaque body be placed so as to intercept a portion of its rays (Fig. 6). If we construct a conical surface touching the body all round, and having its vertex at the luminous point, it is evident that all the space within this surface on the farther side of the opaque
