
INSTEAD OF a pinhole, Russell W. Porter and others have been experimenting with a slit for the knife-edge test, and it appears there is a decided gain in delicacy. Mr. Porter sends in the following description of the new test:
"Here is, so far as I know, something new in knife-edge testing:
"In the last stages of testing an optical surface by the knife-edge method of Foucault, it is often found desirable to increase the contrast of the so-called knife-edge shadows without loss of sensitivity due to enlarging the size of the pinhole.
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