528 PEASE— MEASUREMENT OF STAR DIAMETER. 



fairly close together there appears at the focus of the telsecope a 

 series of interference fringes very sharp and clear cut, superposed 

 on the ordinary diffraction image (Fig. 3) of the star; this image 



Fig. 3. Diagram of star image showing diffraction rings and interference 



fringes. 



is present at all times, even though the fringes may not be visible. 

 It consists of a central bright disk surrounded by alternate bright 

 and dark rings, the relative brightness of which depends upon the 

 size of the openings. In all cases, however, the outer rings are 

 fainter than the central disk. 



As the distance between the slits in front of the telescope is 

 increased, the portions of the wave fronts from any two points 

 of the source passing through them are, owing to their mutual incli- 

 nations, further apart and no longer in phase. Consequently over- 

 lapping of patterns takes place at the focus and a diminution in the 

 relative contrasts of the fringes ensues. As the slits in front of the 

 telescope continue to be separated, a distance is reached where 

 the fringes vanish altogether. It has been found that the angular 

 diameter of an artificial star disk, uniformly illuminated, is equal to 

 1.22 X/h, where A is the wave-length of the light used and h is the 

 distance in question. 



The full curve drawn in Fig. 4 represents the relationship be- 

 tween the visibility or clearness of the fringes and the relative sep- 

 aration of the slits for a given disk and color and it shows that the 

 fringes are brightest when the openings are close together, that it 

 decreases rapidly as the slits are separated and that it is zero when 



