REVERSED AND NON-REVERSED SPECTRA. 



119 



of the diagram. Thus, for instance, the distance 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. The horizontal distribution of 

 dark bands around the center is determined by variations e cos r and is not 

 linear. 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. 



In conclusion, therefore, the main feature in modifying the type of inter- 

 ference pattern is the varying thickness of the compensator. For oval types 

 the preponderating lens is convex; for the hyperbolic type it is concave. 

 Neither of these lenses is here appreciably affected in modifying the horizontal 

 distribution of path-difference, because the dispersion of the grating requires 

 a horizontally parallel system of rays. 



68. Observation with lens systems on both sides. The method shown in 

 plan in figure 85 (L and L' convex lenses, G grating, M and N mirrors, telescope 

 at T} was tested. The outcome can not at once be foreseen, since the focal 

 distances for different colors is different and since slight displacements of 

 either lens must greatly modify the interference pattern. The latter, however, 

 as obtained in every case, proved to be exceedingly fine lines, tipping in the 



