286 ON LIGHT. 



simple negation of that which constitutes the difficulty. 

 On the other hand, such a "crystalline orb" or ''firma 

 ment^'' of solid matter conceived as a hollow shell ol 

 sufficient strength to sustain the internal tension, and 

 filled with a medium attractively, and 7iot repulsively 

 elastic, might realize (without supposing a solid struc- 

 ture in the contained ether) the condition of transverse 

 vibration \ by establishing, ipso facto, lines of tension in 

 every possible direction, along which undulations might 

 be conveyed, like waves along a stretched cord, thus 

 furnishing a fourth hypothesis, which, to those fond of 

 such speculations may afford matter, sui generis, for con- 

 sideration. 



(69.) Tnterfere7ice of the rays of light. There is hardly 

 a more beautiful or a more instructive object in nature 

 than a large well-blown soap-bubble. Whether we con- 

 sider the perfect regularity of its form, illustrating, as it 

 does, in its exact equilibration the great mecha.nical 

 laws to which the sun and planets owe their spherical 

 figure demonstrating, by its resistance to disruption by 

 blasts of wind which distort it, and by its ready and 

 complete resumption of its normal shape on their cessa- 

 tion, the powerful tensile force which holds it together; 

 and proving, by the instantaneous collection of its filmy 

 tissue into water-globules, in the act of bursting, the 

 immense intrinsic energy of that force as compared with 

 gravitation to the mechanician it is fraught with matter 

 of the highest interest. To the photologist, on the other 

 hand, the vivid colours which glitter on its surface afford 

 at once the simplest and most elegant optical illustra- 



