j^44 MATEKIAL THEOEY OF LIGHT. 



wliicli succeed each other tminterruptedly until tlie cause which produced them 

 ceases to act. The first of these two hypotheses seems to have been of very 

 early origin. It received the sanction of Newton, and was made by him the 

 basis of his reasonings in regard to optical phenomena. It is hence commonly 

 called the Newtonian theory. Until an advanced period in the present century 

 it may be said to have been the generally accepted theory. Laplace, in his 

 great work on celestial mechanics, has founded all his investigations in regard 

 to aberration and astronomical refraction upon it. 



Yet it must be admitted by its advocates, if there remain any who adhere to 

 it still, that it presents, even beforQ we follow it into its applications to the ex- 

 planation of the phenomena v/c have described, many serious difBculties. In 

 ihe first place, if light consist of material particles, these particles must be of 

 inconceivable minuteness, or their living force would be sufficient to destroy 

 every structure, no matter how solid or how tenacious it might be, v/hich they 

 should encounter in their flight. A single grain of matter, moving with the 

 velocity of light, would have a quantity of motion equal to that of a cannon ball 

 of 100 pounds weight, moving with the velocity of 1,500 feet per second. But 

 since destructive power is proportioned, not to the quantity of motion, but to 

 the living force, which varies as the square of the velocity, a single grain of 

 matter moving with the velocity of light v/ould have a destructive poAver equal 

 to that of a mass of 3,350 tons moving with the velocity of 1,500 feet. If light 

 be material, therefore, its particles must be many millions of times less in 

 weight than a single grain. "We have no instruments sufficiently delicate to 

 detect a v/eight so minute. Still it would be possible, by optical arrangements, 

 to concentrate many millions of particles upon a single point. Attcmjits have 

 been made to test the question by the use of such expedients. Dr. Priestley, in 

 his History of Light and Colors, describes an experiment in which he directed 

 the light of the sun, by means of a concave mirror having foiir square feet of 

 surface, upon a balance of exceeding delicacy, without producing any sensible 

 impression. The conclusion is expressed in his own words, as follows : " Now 

 the light in the above experiment was collected from a surface of four square 

 feet, which, reflecting only about half what falls on it, the quantity of matter 

 contained in the rays of the sun incident upon a square foot and a half of sur- 

 face in one second of time, ought to be no more than the twelve hundred millionth 

 part of a grain." 



Dr. Priestley does not consider that, in such an experiment, it is the moment, 

 and not the weight, of the particles of light that would be measured. The 

 amount of inertia in any balance, however delicate, is sufficient to render it an 

 instrument not very well adapted to the purpose in view. The presence of the 

 air is also a disadvantage, both on account of its own resistance to motion and 

 on account of the currents created by the heat which attends the direction of 

 the solar focus upon any solid. The following experiment by Mr. Bennet avoids 

 these objections. This brief account is taken from Professor Lloyd's Essay 

 on the IJndulatory Theory, edition of 1857. " A slender straw was suspended 

 horizontally by means of a single fibre of the spider's thread. To one end of 

 this delicately suspended lever was attached a small piece of white paper, and 

 the Avhole was enclosed within 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 cfiect." 

 These results are negative, it is true, but it must be admitted that they are such 

 as to render the truth of the material theory of light in the highest degree 

 improbable. 



Another difficulty in the way of this 'theory is found in the uniformity of 

 velocity with wliich light reaches us from distances all but infinitely unequal, 

 and from luminous bodies of every magnitude. This equality of velocity in the 

 propagation of the light of the stars is evinced in the universality of the law of 



