780 Mr. F. Twyman on an Interferometer 



Thus, in fig. 3 the full lines represent the lens and mirror 

 of the ordinary lens interferometer for axial pencils. The 

 modifications necessary in the relative positions of lens and 



Fig-. 3. 



mirror for pencils oblique to the lens are shown by the broken 

 lines. The mechanism must obviously be such that ITU' = ON. 

 Such, briefly, is the principle on which the camera-lens 

 interferometer is constructed. 



Capacity of the Standard Camera Lens Interferometer. 



The Camera Lens Interferometer to be described will test 

 practically every camera lens from 4 in. (101 mm.) up to 21 in. 

 (686 mm.) focal length. Lenses of 4 in. (101 mm.) focal 

 length can be tested for oblique pencils up to 50° on each 

 side of the axis. The longest focal length which can be 

 tested depends upon the angular range required on each side 

 of the axis, and is as follows : — 



32§ in. lenses can be tested up to an obliquity of 5° 



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