PROPAGATION OF LIGHT. 11 



of light has been ever perceived. The experiments of Mr. Ben- 

 net seem to be decisive on this point. In these experiments a 

 slender straw was suspended horizontally by means of a single 

 fibre of the spider's thread. To one end of this delicately sus- 

 pended lever was attached a small piece of white paper, and 

 the whole was inclosed in a glass vessel, from which the air 

 was withdrawn by the air-pump. The sun's rays were then 

 concentrated by means of a large lens, and suffered to fall 

 upon the paper, but without any perceptible effect. 



(14) But the actual velocity of light is not the only diffi- 

 culty which the theory of emission has to encounter at the 

 very outset. It has been further proved that this velocity is 

 the same, whether the light is directly emitted from the 

 Sun or a fixed star, or reflected from a planet or its satel- 

 lite that it is, in short, independent of the luminous source, 

 as well as of the subsequent modifications which it under- 

 goes in the celestial spaces. It is not easy to account for 

 these facts in the theory of emission. The emissive force, 

 required to produce the known velocity, is calculated to bS 

 more than a million of million times greater than the force 

 of gravity at the earth's surface ; and it can hardly be sup- 

 posed that this prodigious force is the same for all the various 

 and independent bodies of the universe, and that it acts 

 equally on all the particles of light, so as to generate in them 

 the same velocity. Yet even this assumption will not avail. 

 Laplace has shown, that if the diameter of a fixed star were 

 250 times as great as that of our sun, its density being the 

 same, its attraction would be sufficient to destroy the whole 

 momentum of the emitted molecules, and the star would be 

 invisible at great distances. With a smaller mass there will 

 be a proportionate retardation, so that the final velocities will 

 be different, whatever be the initial ones. The suggestion of 

 M. Arago seems to offer the only way of escaping the force 



