NOMOS. 131 



ocean-orb would act, in fact, as a spherical The soiar 

 lens. A similar result would also follow 

 if this orb of water were replaced by an 

 orb of rock-salt, but with this differ ence, 

 that the focus in which the rays converged would 

 coincide with the surface of the sphere, or else fall 

 within that surface. This difference would be owing 

 to the greater refractive powers of the salt as compared 

 with the water, for the focus must be at the surface, or 

 just within the surface, in all diathermanous spheres 

 whose refractive indices are 2, water being 1 C 336. 

 If now a substance of still higher refractive powers 

 were substituted for the rock-salt sphere, the focus 

 would be still deeper within the surface most re- 

 moved from the sun ; and if the powers were very 

 high the focus might even be withdrawn to the 

 centre. If, for example, an orb of chromate of lead 

 (whose refractive index is 3) were substituted for the 

 sphere of rock-salt, the focus would be very con- 

 siderably within the surface. Now the diatherma- 

 nous properties of various bodies vary very consider- 

 ably. Many bodies are altogether athermanous to 

 artificial heat, but all bodies, so far as we know, are 

 more or less diathermanous to solar heat. Trans- 

 parency is not at all necessary to diathermanency. 

 Rock-salt, which is the most diathermanous of all 

 known bodies, transmits the heat with equal readi- 

 ness when its surface is blackened with smoke, and 

 the opacity of obsidian opposes no bar to this trans- 

 mission. What then? Is the Earth herself per- 

 meated by some of the solar rays ? Is " nothing hid 

 from the heat thereof?" If it be so and mere 



K 2 



