Some experiments concerning the counting of scintillations produced by alpha particles.—Part III. Practical applications

Abstract

There are two important characteristics of the microscope or any other optical system used for scintillation counting, which may influence the number observed, namely, the numerical aperture and the magnification. In order to show clearly the role of each factor it seemed desirable to investigate how the percentage of the number of particles observed varied with the numerical aperture in two cases where the magnification was widely different. The first case chosen was the counting of scintillations with a microscope of magnification 50, where the numerical aperture could be varied at will by placing stops on the objective. Stops of black paper which fitted the objective and could be easily interchanged in the dark were used. The numerical aperture corresponding to each objective stop was measured in the usual way (see, for example, ‘Dictionary of Applied Physics,’ vol. 4, p. 205 (1923)). The importance of the numerical aperture is not due to its influence on resolving power, but to its influence on the fraction of the light from a scintillation which enters the objective. From the definition of numerical aperture it follows that the fraction of the light entering the objective from the object viewed is ½ (1 — √ 1— ( n.a ) 2 ).