MOMENTUM OF LIGHT.

109

This observation, which was first made by Roemer, has beenconfirmed by Dr. Bradley's ingenious Theory of the Aberrationof the Lights of the fixed Stars.*

2. MOMENTUM AND MAGNITUDE

OF THE

PARTICLES OF LIGHT.

The velocity of light being thus ascertained, we should beenabled to form some notion of the magnitude of the particles oflight, if we were in possession of good observations on the effectof their momentum.

For example; it is found that a ball from a cannon at its firstdischarge flies with a velocity of about a mile in eight seconds,and would therefore arrive at the sun in thirty-two years, sup-posing it to move with unremitted celerity. Now light movesthrough that space in about eight minutes, which is two millionstimes faster. But the forces with which bodies move are astheir masses multiplied by their velocities: If therefore the par-ticles of light were equal in mass to the two millionth part of agrain of sand, we should be no more able to endure their im-pulse, than that of sand when shot point-blank from the mouthof a cannon.f How much less must therefore be the particlesof light, which makes no sensible impression upon so delicatean organ as the eye.

3. LIGHT MOVES IN STRAIGHT LINES.

Whenever light passes through a space occupied by a mediumof uniform density, it describes right lines in all directions.

* Philosoph. Trans, xxxv. p. 637, and xir. p. 1.

+ Nicholson’s Introd. to Natural Philosophy, vol. I. p. 256.