Six Lectures on Light eBook

This eBook from the Gutenberg Project consists of approximately 228 pages of information about Six Lectures on Light.

Six Lectures on Light eBook

This eBook from the Gutenberg Project consists of approximately 228 pages of information about Six Lectures on Light.
in the smallest fact, a picture of the largest.  He was one day upon the Thames in a boat, and noticed that as long as his course remained unchanged, the vane upon his masthead showed the wind to be blowing constantly in the same direction, but that the wind appeared to vary with every change in the direction of his boat.  ‘Here,’ as Whewell says, ’was the image of his case.  The boat was the earth, moving in its orbit, and the wind was the light of a star.’

We may ask, in passing, what, without the faculty which formed the ‘image,’ would Bradley’s wind and vane have been to him?  A wind and vane, and nothing more.  You will immediately understand the meaning of Bradley’s discovery.  Imagine yourself in a motionless railway-train, with a shower of rain descending vertically downwards.  The moment the train begins to move, the rain-drops begin to slant, and the quicker the motion of the train the greater is the obliquity.  In a precisely similar manner the rays from a star, vertically overhead, are caused to slant by the motion of the earth through space.  Knowing the speed of the train, and the obliquity of the falling rain, the velocity of the drops may be calculated; and knowing the speed of the earth in her orbit, and the obliquity of the rays due to this cause, we can calculate just as easily the velocity of light.  Bradley did this, and the ‘aberration of light,’ as his discovery is called, enabled him to assign to it a velocity almost identical with that deduced by Roemer from a totally different method of observation.  Subsequently Fizeau, and quite recently Cornu, employing not planetary or stellar distances, but simply the breadth of the city of Paris, determined the velocity of light:  while Foucault—­a man of the rarest mechanical genius—­solved the problem without quitting his private room.  Owing to an error in the determination of the earth’s distance from the sun, the velocity assigned to light by both Roemer and Bradley is too great.  With a close approximation to accuracy it may be regarded as 186,000 miles a second.

By Roemer’s discovery, the notion entertained by Descartes, and espoused by Hooke, that light is propagated instantly through space, was overthrown.  But the establishment of its motion through stellar space led to speculations regarding its velocity in transparent terrestrial substances.  The ‘index of refraction’ of a ray passing from air into water is 4/3.  Newton assumed these numbers to mean that the velocity of light in water being 4, its velocity in air is 3; and he deduced the phenomena of refraction from this assumption.  Huyghens took the opposite and truer view.  According to this great man, the velocity of light in water being 3, its velocity in air is 4; but both in Newton’s time and ours the same great principle determined, and determines, the course of light in all cases.  In passing from point to point, whatever be the media in its path, or however it may be refracted or reflected, light takes the course which occupies least time.  Thus in fig. 4, taking its velocity in air and in water into account, the light reaches G from I more rapidly by travelling first to O, and there changing its course, than if it proceeded straight from I to G. This is readily comprehended, because, in the latter case, it would pursue a greater distance through the water, which is the more retarding medium.

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Six Lectures on Light from Project Gutenberg. Public domain.