i88 3 .] 
The Velocity of Light. 51 1 
There is no light without heat, but there is heat without 
light. I have defined heat as the rotary motion or tendency 
of masses and smallest particles, themselves having their 
density unequally distributed in them, and light as their 
orbital motion, dividing in waves. There is no light, only 
heat, — dark light. — when the orbital progressive motion falls 
entirely within the individual, when the transport of its 
centre of force in space is less than the mean velocity by 
rotary motion of any point of the outer surface of the 
particle, or aggregation of particles. 
Emission and undulation theories are not irreconcilable. 
To see this we need not discuss the “ material or immaterial 
agent,” or the ponderable or imponderable ether, or my 
rejection of either. “ One distinguishes two general kinds 
of motion essentially different, the translatory and the vi- 
bratory.” “ The former is the basis of the system of 
emission, the latter of that of undulation of light.” This 
distinction merely indicates differences produced, in equal 
results from equal causes, by the variety of extraneous cir- 
cumstances in which they are placed and under which they 
occur. Both motions are due to an attraction and repulsion 
of one mass by another, both these effects being convertible 
into each other. The difference consists in the rapid suc- 
cession of reversions, and the contact or approximate contact 
of the individual masses and particles. 
Matter from space, meteors of all sizes and kinds join 
the earth, and a regular interchange of mass exists within 
the solar system, and more or less between systems. We 
may therefore advance in favour of the emission theory the 
hypothesis that Crookes’s corpuscles are propelled from the 
sun and stars with the velocity of light towards the earth, 
attracted, directed, and concentrated by her through a polar 
interaction of projector and receiver, without there being 
any need for a fourth state of matter of which we have no 
concept. 
I call .the aCtion polar because it cannot exist outside the 
profiles of projector and receiver, without whose attraction 
the centrifugal energy of the projector, or particles of his, 
would not acquire the trajectory surplus necessary to start 
particles away from its own re-attraCtion and polar aCtion, 
the mutual attraction and energy between both masses being 
zero outside the cones connecting the semi-surfaces of gene- 
rator and receiver, in so far as there is no third centre of 
force affeCting both. 
Striking the atmosphere, meeting with resistance, the 
gradually penetrating projectiles produce light and radiant 
