118 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



If there are differences in thickness of the air-films, the resulting 

 differences in the height of the plates will be less in the ratio tan «. 



B 



Figure 5. 



An error of \jn may be admitted for each plate, — even in the most 

 unfavorable case in which the errors all add ; and consequently the 

 admissible errors in the thickness of the air-films may be of the order 

 A/^^«. For instance, for 20 plates the average error may be a whole 

 wave-length if the inclination a is o'^ . As there is always a more or 



FiGUKE 6. 



less perfect compensation of the errors, the number of plates or the 

 inclination may be correspondingly greater. Accordingly, it may be 

 possible to make use of 50 elements, and the plane may be inclined at 

 an angle of 20° to 30°. It would be necessary in this case, however, 

 to use a rather large objective. Possibly this may be avoided by cutting 

 the surface ^ to a spherical curvature, thus forming a sort of concave 

 echelon. 



