THE IMMERSION APERTURE. 79 



product of the sines and the refractive indices, i.e. 

 (n Sin w 2 ), for the square of the numerical aperture. 



The fact is therefore established that the aperture o{ 

 a dry objective of 180 does not represent, as was sup. 

 posed, a maximum, but that aperture increases with the 

 increase in the refractive index of the immersion fluid ; 

 and it should be borne in mind that this result has 

 been arrived at in strict accordance with the ordinary 

 propositions of geometrical optics, and without any 

 reference to or deductions from the diffraction theory 

 of Prof. Abbe. 



We have, however, still to determine the proper 

 function of aperture, immersion objectives of large 

 aperture excelling, as is well known, any dry objective 

 in the delineation of minute structures. 



The old explanation of the increased power of vision 

 obtained by increase of aperture was, that by the 

 greater obliquity of the rays to the object " shadow 

 effects " were produced, a view which overlooked the 

 fact, first, that the utilization of increased aperture 

 depends not on the obliquity of the rays to the object, 

 but to bhe axis of the microscope ; and secondly, that just 

 as there is no acoustic shadow produced by an obstacle, 

 which is only a few multiples of the length of the 

 sound waves, so there can be no shadow produced by 

 minute objects which are only a few multiples of the 

 light waves, the latter then passing completely round 

 the object. The Abbe diffraction theory, however, 

 supplies the true explanation, and shows that the 

 increased performance of immersion objectives of 

 large aperture is directly connected (as might havt 

 been anticipated) with the larger "openings " in the 

 proper sense of the term, which, as we have just 

 seen, such objectives necessarily possess. It has also 

 been shown, then, in order that the image should exactly 

 correspond with the object, all the diffracted rays 

 must be gathered up by the objective. If any ar^ lost 

 we then get not an image of the real object but a 

 spurious one. Now, if we have a coarse object, the 

 diffracted rays are all comprised within a narrow cone 

 round the direct beam, and an objective of small 



