Page:Light waves and their uses.djvu/62

 LECTURE III

APPLICATION OF INTERFERENCE METHODS TO MEASUREMENTS OF DISTANCES AND ANGLES

the last lecture we considered the limitations of the telescope and microscope when used as measuring instruments, and showed how they may be transformed so that the diffraction and interference fringes which place the limit upon their resolving power may be made use of to increase the accuracy of measurements of length and of angle. We have named these new forms of instrument interferometers and illustrated many of the forms in which they may be made.

It has been found that the particular form of interferometer described on p. 40 is the most generally useful, and the principal subject of this lecture will be to illustrate the applications which have already been made of this instrument.

But before passing to the first application of the interferometer, we may make a little digression, and consider briefly the two theories which have been proposed to account for the various phenomena of light. One of these is the undulatory theory, which has already been explained; the other is the corpuscular theory, which for a long time held its ground against the undulatory theory, principally in consequence of the support of Newton.

The corpuscular theory supposes that a luminous body shines in virtue of the emission of minute particles. These corpuscles are shot out in all directions, and are supposed to produce the sensation of vision when they strike the retina. The corpuscular theory was for a long time felt to be unsatisfactory because, whenever a new fact regarding light was discovered, it was always necessary to make some