136 NOMOS. 



brought into a focus beyond its centre ; and there is 

 an equal want of reason why these rays may not 

 travel with extreme rapidity ; and if so, then we may 

 assume two things. 



If the solar rays are brought to a focus deep within 

 the earth, we may assume, in the first place, that the 

 parts corresponding to the focus must be 

 fused ; for cornelian, agate, and rock crystal 



soarray h s e were readily fused in the focus of Parker's 

 win be to great lens a lens whose diameter was 32^- 



cause a per- 



manentbuig- inches, and whose focal length was 6 feet 



ing out of the _. . , mi * j. I, 



equatorial 8 inches. This fusion, moreover, must be 

 region or the atten ^ e( j ^fa an equivalent degree of 



expansion, and this expansion must neces- 

 sitate a bulging out in the overlying parts of the 

 earth. It follows, also, that the position of the 

 focus must continually change as the earth revolves 



portion as the attracting powers of the currents which pass through 

 the centre preponderate over those of the currents which pass to- 

 wards the edge. It is easy to understand, indeed, that the approxi- 

 mation of the focus to the centre must be directly proportionate to 

 the convexity of the lens on the one hand, and, on the other hand, 

 to the attracting power of the individual currents, for in lenses of 

 different substances the relative power of the individual currents 

 may, or rather must, be different. 



And if this be so, it follows that a contrary result must happen 

 when the currents have to pass through a con- 

 cave Jens. Let the diagram represent such a 

 lens, and let the currents abed and e be 

 the currents acted upon by it, and it is at once 

 evident that the event which happened in the 

 Fig. 28. last instance must be reversed, and the result 



be divergence instead of convergence. This 

 must be evident after what has been said, and no further explanation 

 is therefore necessary. 



