526 



PEASE— MEASUREMENT OF STAR DIAMETER. 



intersect, say at a screen E. Having left B at the same instant, the 

 two pencils arrive at the screen E on the axis at the same time, the 

 crest or trough of one wave falling upon the corresponding crest or 



Interference of two light pencils. 



trough of the other wave, each thus reinfocing the other. At F , F 

 are points on either side of the axis where the crest from one wave 

 falls in the trough of the other; the result is the complete neutrali- 

 zation of the one by the other (and the total absence of motion) 

 and consequently darkness at that point. Between these two points 

 there is a gradual change in intensity, so that one grades into the 

 other. The observed result is a series of parallel bands, alternately 

 bright and dark, lying on both sides of the axis, bright wherever the 

 path difference from the screen E to the two apertures is equal 

 to any number of even half wave-lengths of light as 2, 4, 6, etc., 

 and dark when the number of half waves in the path difference is 

 an odd number as i, 3, 5. No light is lost in the mutual action of 

 the two pencils on one another; it is simply redistributed, and 

 what is removed from the dark region is to be found in the brighter 

 portion. 



Fresnel improved the method of observing the bands by setting 



