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Page 354 - You find that fewer soldiers now pass you, and that the interval between each is longer. Now suppose yourself at rest, and suppose the barrack to have a motion now towards you, now from you. In the first case the men will be paid out, so to speak, more rapidly. The motion of the barrack-gate towards you will plant each soldier nearer the preceding one than he would have been if the barrack had remained at rest. The soldiers will really be nearer together. In the second case, it is obvious that the...
Page 334 - ... spectroscope appear much brighter than the corresponding parts of the spectrum of the more lustrous solid body. Now here comes a very important point : supposing the continuous spectrum of a solid or liquid to be mixed with the discontinuous spectrum of a gas, we can, by increasing the number of prisms in a spectroscope, dilute the continuous spectrum of the solid or liquid body very much indeed, and the dispersion will not seemingly reduce the brilliancy of the lines given out by the gas ; as...
Page 334 - I am sure know, is scattered broadcast, so to speak, by the prism into a long band of light, called a continuous spectrum, because from one end of it to the other the light is persistent. The light from gaseous and vaporous bodies, on the contrary, is most brilliant in a few channels ; it is husbanded, and, instead of being scattered broadcast over a long band, is limited to a few lines in the band — in some cases to a very few lines. Hence, if we have two bodies, one solid or liquid and the other...
Page 355 - Next fix your attention on the edge of the globe — the limb, in astronomical language ; here it is evident that an upward or downward movement is as powerless to alter the wave-length as a lateral movement was in the other case, but that, should any lateral or cyclonic movement occur here of sufficient velocity, it might be detected. So that we have the centre of the disc for studying upward and downward movements, and the limb for studying lateral or cyclonic movements, if they exist. If the hydrogen-lines...
Page 318 - ... different for a different form of the curve. May not the colours of the fixed stars be owing to a difference of this kind? white will acquire a tinge of yellow ; if the blue and green be successively stopped, this yellow will grow more and more ruddy, and pass through orange to scarlet and blood red. If, on the other hand, the red end of the spectrum be stopped, and more and more of the less refrangible portion thus successively abstracted from the beam, the white will pass first into pale, and...
Page 354 - Now let the barrack represent the hydrogen on the sun perpetually paying out waves of light, and the elastic cord represent one of these waves ; its length will be changed if the hydrogen and the eye approach each other by the motion of either. Particular wave-lengths with the normal velocity of light are represented to us by different colours. The long waves are red. The short waves are violet. Now let us fix our attention on the green wave, the refrangibility of which is indicated by the F line...
Page 433 - What, then, was the cause of the descent ? Simply this. The lead was exposed to the varying temperatures of day and night. During the day the heat imparted to it caused it to expand. Had it lain upon a horizontal surface, it would have expanded...
Page 332 - ... or both of those fundamental modes, if some of the incident light is of one or other of their periods, or some of one and some of the other ; so that the energy of the waves of those particular qualities of light is converted into thermal vibrations of the medium and dispersed in all directions, while light of all other qualities, even though very nearly agreeing with them, is transmitted with comparatively no loss. (5) That Fraunhofer's double dark line D of solar and stellar spectra is due...