. Light, a textbook for students who have had one year of physics. li-mator is a tube, atone end of which isan achromatic lens, atthe other end a fineslit. The latter iscarefully adjusted atthe principal focus ofFigure 61 the former, so that light which enters the slit and passes through the lens emergesin accurately plane waves. The beam then passes through theprism and is dispersed, that is, the dliferent wavelengths aredeviated to different amounts. It is best to have the prismturned so that the region of the spectrum to be examinedtraverses it at minimum deviation. It should be borne in mi
. Light, a textbook for students who have had one year of physics. li-mator is a tube, atone end of which isan achromatic lens, atthe other end a fineslit. The latter iscarefully adjusted atthe principal focus ofFigure 61 the former, so that light which enters the slit and passes through the lens emergesin accurately plane waves. The beam then passes through theprism and is dispersed, that is, the dliferent wavelengths aredeviated to different amounts. It is best to have the prismturned so that the region of the spectrum to be examinedtraverses it at minimum deviation. It should be borne in mind that when the light leaves theprism although the different wavelengths take different direc-tions, yet all the rays of any one wavelength remain parallelto one another till they strike the objective of the the latter converges the waves of each particularlength to a definite place in the principal foeal plane. Con-nently, there will be in this plane an image of the slit foreach particular wavelength that enters the slit, and the whole (HO). BRIGHT-LINE SPECTRA 111 array of these images constitute the spectrum of the light inquestion. The eyepiece of the telescope forms an enlarged vir-tual image of the spectrum, just as it. would form a virtualimage of any very distant object which the objective fooussedin its focal plane. A spectrometer is a spectroscope provided with a largedivided circle, so that the angular position of the prism, of thetelescope, or of both, can be accurately measured. There isalso a crosshair at the principal focus of the telescope. Thespectrometer is used for making accurate measurements of therefractive indices of prisms, and for other angular measure-ments. A spectrograph is a spectroscope so arranged that thespectrum can be photographed instead of viewed directly. Anyspectroscope can be converted into a spectrograph by removingthe eyepiece from tbe telescope and putting a photographicplate in tbe focal plane of the objective; but
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectlight, bookyear1921
