for Testing Camera Lenses. 



787 



way. For this reason we give in fig. 11 (PL XXIV.) repro- 

 ductions of sketches of the interference appearances corre- 

 sponding to fairly pure examples of these five aberrations. 

 A short consideration of this figure will readily enable the 

 observer to state on inspection the aberrations of the lens he 

 has under test in terms of wave-length and to recognize 

 the outstanding characteristic in any particular case. 



To make a rapid translation of these interferometer pictures 

 into the terms of geometric optics requires a little practice. 

 But an observer, bearing in mind that these pictures are 

 contour maps of wave surfaces, and working continuously 

 for a few days with such an interferometer, will at the end 

 of that time be able almost without thought to form a 

 mental picture of the focus corresponding to the interference 

 pattern. 



Fig. 12. 



b A 



If a quantitative statement of the aberrations is required 

 in terms of ordinary geometrical optics, we can proceed in 

 several ways. If the aberrations of wave-front amount to 

 three or four wave-lengths or more, as in the case of existing 

 camera lenses they always do, one may count or estimate the 

 bands per inch at whatever point of the lens we desire to 

 find the direction of the ray through it, which immediately 

 gives us the angular aberration ; or we may make use of the 

 deflectors described below. For this latter method one 

 requires not only to be able to rotate the deflectors relative 

 to each other in opposite directions as is provided for in the 

 standard instrument, but also to rotate the pair together. In 

 this way one can impose on the incident wave- front a 

 deviation of any desired amount in any direction. We can 

 under these circumstances broaden out the basis at any point 



