6~o 



THE POPULAR SCIENCE MONTHLY. 



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 prog- 

 ress 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 ser- 

 vice, 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. 



FOBMATION 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, al- 

 though 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 



