Einstein's theory demanded the existence of tive. phenomena which could not be explained by Newtonian -gravitation: a slow revolution in the orbit of Mercury, an apparent. displacement of the) stars in the vicinity of the eclipse: ed sun and a shift in the spectral Hines of the sun and stars. The existence -of the first. had been known, but not explained, since the middle of the 19th century; the aveond was looked for and demonstrated "beyond all doubt in 1919 and 1022; but the third was Jong sought for in vain. The results of different. observers were completely contradictory. At Inst, however, the discordances have Einstein shift has a real existe fence.
The spectrum of an are-lamp is continuous band of "rainbow" colours stretching from red at one end to violet at the other. The introduction into the are chemical element produces " line, or group lines crossing the band transversely and occupying posi tions which are always constant for the same clement " under the game conditions, Contain things produce slight changes in the posi-Hons of the lines, ins a contin. ous motion to or away from the observer of the source emitting the ines produces a shift, which is towards the violet if the source!
approaching, and towards the red if it is receding. 'This pheno. ninon, known as the Doppler fleet, oes not-occur if the source is travelling at right angles to the: Tine joining it. to. the observer; if, that' la to say, it is neither ap: proaching nov receding, The: Amount of the shift gives a mea-auto of the velocity of approach or recession: A shift towards the "fod in alto produced by high pres: et sitter 70TH HED, According to Rinstoin, the'speotral lines "of the. gun and stars should show a displacement to the rod, in addition to any that may be due to- motion or preassure, His theory. requires that the lines of fan clement should be shifted more and more to the red og the gravitational field surrounding the clement inerexsos. Thus in amas sive body like the sun, where the ravitational field is hstense, the ines should be shifted to the red sas compared with the same lines produced in the laboratory. The Amount of this shift can be cal-
The problem of deciding when thor this shift is, or is not, pre:
the solar spectrum is; ted by the pre: which can Seounte, pind mers] doubtful The who have done most towards the deciding of this question are Mr. Evershed, director of the Ked! Kanal Observatory, and Dr. St.
Text read from the printed page and cleaned; old type leaves the occasional error.