Light; a course of experimental optics, chiefly with the lantern . ch other in another way. Let apendulum be mounted as in Fig. 171, so that it swings ongymbals, G, from two axes at right angles to each other; ^ ifswung on one axis the bob will vibrate in the path a b ; if onthe other, in the path c d at right a,ngles with it. Let these ^ I give this as conveying a rough, popular idea; in reality, theoriginal ray is divided into two at the first reflection, of which one isretarded J of a vi^ave-length, and still more differentiated to a quarter-vibration by the second, reflection. The rest fol
Light; a course of experimental optics, chiefly with the lantern . ch other in another way. Let apendulum be mounted as in Fig. 171, so that it swings ongymbals, G, from two axes at right angles to each other; ^ ifswung on one axis the bob will vibrate in the path a b ; if onthe other, in the path c d at right a,ngles with it. Let these ^ I give this as conveying a rough, popular idea; in reality, theoriginal ray is divided into two at the first reflection, of which one isretarded J of a vi^ave-length, and still more differentiated to a quarter-vibration by the second, reflection. The rest follows as in the nextmethod. See for details Lloyds Lectures on the Wave Theory. ^ The illustration in this form is, I believe, first due to ProfessorBaden-Powell. 296 LIGHT. [CHAPv represent the two planes of plane polarisation, arid the bopa molecule of ether, and let it have arrived at b in the planeorbit A B. It has therefore reached the limit of its swing?and the next moment will begin to swing back, but at thismoment has no motion. Just at that moment, then,. Fig. 171.—Coipposition of a Circular Vibration. imagine the bob of a duplicate pendulum, moving in anorbit at right angles to a b, and in the exact middle, or fiillpower of its swing, to strike against it, as represented by th^arrow c d. This second bob will yield up its motion andcome to rest, and may be withdrawn; but its. transferred XIV.] QUARTER-WAVE PLATES. 297 force, thus applied tangeritially, will be compounded withthe other, and drive the hob, b, into a new circular orbit,B o D, in the direction of the arrow. Now we see at a glance that the second vibration, to dothis, must be exactly a quarter of a whole or double vibrationbefore or behind the other, or must be at its fullest powerwhen the other is at the moment of rest; and it follows,therefore, that supposing us to be dealing with actualphysical realities—real atoms of ether vibrating in realpaths—a circularly-polarised ray ought to result, if wecaus
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