(226) 
damental quantities for the present simulated test, the tests will certainly be 
more sensitive, will embody a definite fundamental concept of lens performance, 
and the results will be applicable to circumstances other than those actually 
prevailing at the time of the test. The results will probably give information 
more pertinent regarding the performance of the lens and will be more useful 
to the lens designer. Such an improved procedure will be of advantage to the 
lens user in that he will receive more complete and precise information about 
lens performance. It will also be of great advantage to a testing organization 
that is regularly testing lenses for photogrammetric and other purposes in that 
the results of the tests are more generally applicable and of more permanent 
value. 
Because of lack of information I believe there is nothing that can be done 
in the immediate present better than to prescribe certain definite procedures 
for measuring the resolving power of lens and emulsion in combination as has 
been done. However, it should be recalled that a measurement of the resolving 
power with a photographic emulsion as the sensing unit was adopted some 
years ago because at that time it was the most convenient means at hand. 
Since that time the photomultiplier tube has become available and it should 
probably be substituted for the photographic emulsion as a sensing element 
for testing lenses because its response is substantially linear over the intensities 
and frequencies that are to be studied and also because it can be used under 
conditions which give it so high a resolving power that the measurements 
made on the lens will not be influenced by limitations of the detector system. 
Many different methods of applying the photomultiplier tube have been 
developed. Schade of RCA has proposed a measure of lens performance by 
electronic means which depends upon the frequency response, and has devised 
an instrument which makes this measurement in a most convenient manner. 
Hopkins, of the Institute of Optics at Rochester, has proposed as a measure 
the radius of a circle which contains a specified percentage of the total image 
forming light and has made a study of the correlation between this value and 
the quality of the photographs made with a lens. A measure of the gradient at 
the edge of the image of an opaque diaphragm suggests itself. Ingelstam of the 
Optical Laboratory of Sweden has devised an image scanning device so sensitive 
that it can measure the energy within an aerial image transmitted by a slit one 
micron wide. These methods, most of them fully developed and ready for use, 
have been available in perfected form only in the last few years and they offer 
the opportunity for great changes in the methods of lens testing. 
Nevertheless, for the immediate present, the National Bureau of Stan 
dards, as will be reported more fully by Dr. Washer, is making available a 
newly designed resolving power chart which will be produced in two contrasts. 
It is considered that the design of any resolving power chart is to a large extent 
arbitrary, and among the advantages of this chart are the practical ones of 
general availability in a quality which will be precisely controlled over a period 
of years and a design which can be easily read quantitatively and objectively 
by a recording microdensitometer. 
However, it seems probable that the detailed formulation of any resolving 
power test is essentially a stop-gap measure because such a measurement is not 
fundamental in nature. I think we should look forward to measuring some 
fundamental property of a lens, probably by an electronic method; and then