Light as Vibrations in the Aether

Hooke believed that light consists in vibrations of an aether.

He rejected the following:

1. The incompetence of the wave-theory to account for the rectilinear propagation of light
2. Its inability to embrace the polarization discovered by Huygens

This polarization was first interpreted correctly by Newton himself.

On the whole, Hooke favoured a different scheme below[35].

All space is permeated by an elastic aether which is capable of propagating vibrations in the same way as the air propagates the vibrations of sound, but with far greater speed.

This aether pervades the pores of all material bodies. It is the cause of their cohesion. Its density varies from one body to another, being greatest in the free interplanetary spaces.

It is not necessarily a single uniform substance. But just as air contains aqueous vapour, so the aether may contain various “aethereal spirits,” adapted to produce the phenomena of electricity, magnetism, and gravitation.

The vibrations of the aether cannot be supposed in themselves to constitute light.

Light is therefore taken to be something of a different kind, propagated from lucid bodies.

Light is an aggregate of various peripatetic qualities.

Others may suppose light as multitudes of unimaginable small and swift corpuscles of various sizes. These spring from shining bodies at great distances one after another. But these have no sensible interval of time.

These are continually urged forward by a principle of motion which in the beginning accelerates them until the resistance of the aethereal medium equals the force of that principle. This is similar to how bodies that fall in water are accelerated till the resistance of the water equals the force of gravity.

But light is not like any other corporeal emanation.

To avoid dispute and make this hypothesis general, light consists of rays differing from one another in contingent circumstances, as bigness, form, or vigour."[36]

In any case, light and aether are capable of mutual interaction. Aether is the intermediary between light and ponderable matter*.

When ray of light meets a stratum of aether denser or rarer than that through which it has lately been passing, it is, in general, deflected from its rectilinear course.

Differences of density of the aether between one material medium and another account on these principles for the reflexion and refraction of light.

The condensation or rarefaction of the aether due to a material body extends to some little distance from the surface of the body, so that the inflexion due to it is really continuous and not abrupt.

This further explains diffraction, which Newton took to be “only a new kind of refraction, caused, perhaps, by the external aether’s beginning to grow rarer a little before it came at the opake body, than it was in free spaces.”

The regular vibrations of Newton’s aether were not supposed to constitute light.

• But its irregular turbulence represented fairly closely his conception of heat.

Newton supposed that when light is absorbed by a material body, vibrations are set up in the aether, and are recognizable as the heat which is always generated in such cases.

He conceived that the conduction of heat from hot bodies to contiguous cold ones were caused by vibrations of the aether propagated between them.

The violent agitation of aethereal motions excites incandescent substances to emit light.

Newton assumed that light is not actually made up of the vibrations of an aether. But such vibrations may exist in close connection with light.

This easily leads to the idea that rays of light are streams of corpuscles emitted by luminous bodies.

This was not the hypothesis of Descartes himself. But it was so thoroughly akin to his general scheme.

The scientific men of Newton’s generation were mostly deeply imbued with the Cartesian philosophy.

They instinctively selected it from the wide choice of hypotheses which Newton had offered them.

• Later writers associated it with Newton.

A phenomenon known for nearly 50 years made Newton’s philosophy favored over Descartes.

Vincenzo Cascariolo was a shoemaker of Bologna.

In around 1630, he had discovered a substance later called Bologna stone or Bologna phosphorus. It could shine in the dark after being exposed for some time to sunlight.

This storage of light was most easily explained by the corpuscular theory.

However, it was found that phosphorescent substances do not necessarily emit the same kind of light that was used to stimulate them.

Newton considered colour to be an inherent characteristic of light.

He inferred that it is associated with some definite quality of the corpuscles or aether-vibrations.

The corpuscles corresponding to different colours would excite vibrations of different types in the aether, like sonorous bodies of different pitch.

That was the first enunciation of the great principle that:

• homogeneous light is essentially periodic
• differences of period correspond to differences of colour.

The analogy with Sound is obvious.

Newton developed the theory of periodic vibrations in an elastic medium[38] in connection with the explanation of Sound. This was a very influential idea.

Newton gave attention to the colours of thin plates. He determined the empirical laws of the phenomena with great accuracy.

He supposed that:

The interval between two consecutive dispositions to easy transmission, or “length of fit,” he supposed to depend on the colour, being greatest for red light and least for violet.

If a ray of homogeneous light falls on a thin plate, its fortunes as regards transmission and reflection at the two surfaces will depend on the relation which the length of fit bears to the thickness of the plate.

On this basis, he built up a theory of the colours of thin plates.

Newton’s “length of fit” corresponds in some measure to the quantity which in the undulatory theory is called the ‘wavelength of the light’.

But the suppositions of easy transmission and reflection were soon found inadequate to explain all Newton’s experimental results, at least without making other and more complicated additional assumptions.

The Speed of Light

At the time of the publication of Hooke’s Micrographia, and Newton’s theory of colours, it was not known whether light is propagated instantly or not.

Galileo tried to answer this experimentally many years previously[40].

He stationed 2 men with lanterns at a considerable distance from each other.

• One of them was directed to observe when the other uncovered his light, and exhibit his own the moment he perceived it.

But the timespan required by the light for its journey was too small to be perceived in this way.

The discovery was ultimately made in 1675 by an astronomer, Olof Roemer[41] (b. 1644, d. 1710).

He observed that the eclipses of the first moons of Jupiter were apparently affected by an unknown disturbing cause.

The time of the occurrence of the phenomenon was retarded when the earth and Jupiter, in the course of their orbital motions, were farthest from each other. It was accelerated when they were nearest.

Roemer explained this by supposing that light requires a finite time to travel from that moon to the earth.

By observing eclipses, he calculated the timespan required for its passage from the sun to the earth (the light-equation, as it is called) was 11 minutes.[42]