MOUNT WILSON OBSERVATORY. 279 



Calibration of Visibiuty of Fringes. 



In the hope of extending the possibilities of the stellar interferometer 

 to stars whose angular diameter is too small to admit of observing the 

 disappearance of the interference fringes, Professor Michelson, with 

 the assistance of Mr. Pease, has made use of the device to which brief 

 reference was made by him in his description of the instrument. For 

 this purpose it is necessary, in the case of stars having a diameter 

 sufficient to cause a distinct diminution in the visibility, to measure 

 the visibility as a function of the distance between the outer mirrors. 

 The diameter may then be calculated with an order of accuracy pro- 

 portional to the accuracy of this measurement. 



The method consists in the use of an auxiliary' interferometer with a 

 small fixed distance between the outer mirrors. A comparison system 

 of fringes then appears in the focal plane of the observing telescope, 

 in the present case the 100-inch reflector, and the visibility of these 

 fringes may be altered in known ratio by altering the relative areas of 

 the two apertures through which the auxiliary pencils of light from the 

 star pass. If r is the ratio of the a 'eas of these apertures, the visibility 

 of the frmges is given by the expression 



r-f- 

 r 



After the two systems of fringes have been brought to equality, the 

 stellar diameter can be calculated from the formula already given by 

 Professor Michelson. 



In the first trial of this method the apertures were squares 4.75 

 inches on a side, whoie effective areas were varied by a sliding screen 

 which covered oae aperture as the other was uncovered. The ratio of 



effective areas accordingly is proportional to — , s being the 



4.75 — s 



width of one of the apertures. A number of observations showed 

 the practicability of this plan, but presented the difficulty that when 

 one of the apertures is nearly closed the light is drawn out by dif- 

 fraction into a band which disturbs proper estimates of visibility. 

 The use of a number of apertures was next tried, thus leaving the 

 total intensity constant without altering the dimensions of the central 

 image. A number of measurements was made in this way, but the 

 total intensity of the light in the comparison fringes was found to be 

 too small for work upon any but the very brightest stars. Accord- 

 ingly it was decided to return to the original plan, but at the sugges- 

 tion of Mr. Pease the modification was introduced of varying the 

 area of only one aperture and keeping its form the same, whether 

 square or circular. 



With this arrangement the total intensity is not constant, but is 

 greater than with the preceding forms of aperture. A series of obser- 



