304: 



G. Barus — Use of Compensators. 



Suppose furthermore the same phenomenon is exhibited in 

 terms of wave length \, as in the lower part of the diagram, 

 the spectrum being equally wide for all values of y, while at 

 any given y, the upper diagram still shows the number of dark 

 points or bands between r and v. If now we suppose that 

 under any conditions these dark points are grouped symmetric- 

 ally with reference to any given color (which is probable, for 

 a maximum or a minimum of any value of y will be so for all 



Fig. 3. 



values) and that the successive dark points have been connected 

 by a curve, the interference pattern will be of the elliptic type 

 in case of aa', a" a'", and of the hyperbolic in the case of a'a". 

 The other features of the phenomenon are secondary and 

 therefore left out of the diagram. Thus, for instance, the dis- 

 tance apart of the bands shrinks from red to violet and the 

 ovals, etc., are only appreciably symmetric because they occupy 

 so small a part of the spectrum. Whether the long axes of the 

 ellipses are horizontal or vertical depends upon the slope of the 

 lines r and v. Maxima and minima will not, as a rule, occur 

 close together, though in certain wood-grain shaped patterns 

 this seems to be the case. 



