Abstract
WHILE demonstrating the interference patterns of various lines by means of a Hilger C.D. instrument in conjunction with a Lummer-Gehrcke plate, a Philips 'Philora' sodium vapour lamp was used to produce the patterns of the sodium, doublet λ 5890/96. This lamp is in the form of a U-tube enclosed in an outer vacuum jacket and is excited by 230 V. A.C. mains through the usual electric control gear. The lamp usually takes about 10–12 minutes to reach its maximum intensity. In the first few minutes, each, of the two lines showed its normal monochromatic interference pattern as sharp fringes; but as the lamp got heated and acquired its steady state of illumination, the fringe system of both the lines presented a complex appearance. To follow this development of complexity, the lamp was switched off for some time to cool down and then restarted. The fringe pattern was viewed through the telescope from the time it was switched on. As the lamp gained its full wattage (65 W.), each of the fringes in the two wave-lengths began to broaden out and ultimately developed almost symmetrical splitting into two defined components. The magnitude of the actual splitting computed from measurement is, within the limits of experimental errors, 0·064 A. for λ 5890 and 0·053 A. for λ 5896.
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