PEASE— MEASUREMENT OF STAR DIAMETER. 



527 



two prisms of equal angle side by side as indicated in Fig. 2. 

 Light from a source G passing through the two prisms arrives at 

 a screen H, and here the same principle of crest on crest or crest 

 on trough produces interference fringes. It should be borne in 



FRE5NEL BIPR15M, 



SOURCE 



51PRI5M 



B = E>R16HT 5AND 

 D^PARK 5AMD 

 Fig. 2. Fresnel biprism. 



5CREEN 



mind that light from two separate sources cannot interfere ; it is only 

 two pencils of light from the same source that produce the phenome- 

 non in question. As the size of the source of light is slightly increased, 

 each point in it produces a series of concentric spherical waves 

 and there are therefore at the screen a great many overlapping 

 interference fringe patterns slightly displaced with respect to one 

 another, which reduce the contrast between the dark and light bands. 

 Without going into detail, it is found mathematically and experi- 

 mentally that for a given distance between the slits there is a certain 

 size of source for which this overlapping is complete, fringes are 

 not visible and ordinary illumination takes place. As the source is 

 slightly increased in size the fringes reappear much less conspicuous 

 than before and then vanish again. 



In the application of this principle to the telescope, the wave 

 front from a distant star is considered a plane although it is ac- 

 tually a portion of a sphere. Each point of the star produces a 

 wave which is inclined slightly to that from the central point. All 

 these waves superimpose in apparently one wave front on the axis 

 of the telescope and for a distance of a foot or two on either side. 

 When the telescope is covered with a screen having two apertures 



