as shown by the solid lines in fig5 and the size of a second order light source image becomes ip 1 thereby increasing the numerical aperture on the other hand where the concave lens 50 and the convex lens 52 are interchanged with each other as shown in fig6 the illuminating light beam becomes as shown by the solid lines in fig6 and the size of a second order light source image becomes ip 2 thereby producing the same condition as if the numerical aperture were decreased by using these arrangements it is possible to vary the u03c3 value of the illuminating optical system without using the aperture stop 18 and this has the effect of preventing any vignetting and improving the illumination efficiency if it is desired to vary the o value continuously this can be accomplished by arranging a zoom lens system in front of the fly eye lens 16 instead of storing the relation between the numerical aperture and the time constant in the form of table 1 in the memory 42 it is possible to approximate the relation between the numerical aperture and the time constant with a suitable function e g n order function as shown in fig7 and store the function in the memory 42 then the corresponding time constant will be determined by calculation from the preset numerical aperture in this case the selector 40 functions.