PEASE— MEASUREMExNT OF STAR DIAMETER. 525 



ment. Stephan had already found fringes with apertures separated 

 by 25.6 inches ; Alichelson found the same result in turn, first with 

 the full aperture of the 40-inch Yerkes Refractor, then with the 

 60-inch reflector at IMount Wilson and finally the 100-inch Hooker 

 Reflector, even though the seeing was not very good. The im- 

 mediate result was the application of interference methods to the 

 measurement of double stars, and Anderson developed a method by 

 means of which he determined the distance between the components 

 of Capella with a very great degree of precision, it being impossible 

 to do this by ordinary methods on account of the closeness of the 

 companions to one another. The next step undertaken was to adapt 

 the great reflector in accordance with Michelson's plan of 1890, by 

 mounting four auxiliary small mirrors on a beam placed across 

 the upper end of the telescope tube. Success attended its installa- 

 tion and in August, 1920, the interference fringes obtained 

 on Vega with an aperture equivalent to 18 feet were as easily 

 seen as those at 6 feet. 



Meanwhile, Eddington, Russell, Shapley, and others had made 

 calculations of the diameters of a number of stars based on estimates 

 of their surface brightness, and their results indicated that a Orionis 

 would be an excellent object for an attempt to measure the diameter. 

 Merrill first examined the star with the apparatus used by Anderson 

 in the measurement of Capella and found a definite decrease in the 

 visibility of the interference fringes with the slits separated the full 

 aperture of the mirror. An actual measurement of the diameter of 

 a Orionis was then made by the writer on Dec. 13, 1920; a descrip- 

 tion of the instrument and method is given below. 



Before describing the 20-foot interferometer in detail it may 

 be well to recall a few of the principles of interferometry. Thomas 

 Young found that two pencils of light from a point source, when 

 brought together again, can be made to produce interference bands 

 or "fringes." A pinhole in a screen A (Fig. i") in front of a 

 candle will, for laboratory experiments, serve as a source of light. 

 Light spreads from this pinhole in concentric spherical waves and 

 IS intercepted by a screen B, having in it two pinholes equidistant 

 from the axis so that AC = AD. These openings furnish the two 

 pencils of light and interference takes place where the wave fronts 



