HISTOLOGICAL ADVANTAGE OF LARGE APERTURE 373 



every cell. The circular elastic tissue is also displayed, while the 

 whole image has an increased sharpness and perfection. 



Thus, with the objective (A -20 X.A. = frds inch) of lower 

 aperture, the endothelium cells can be seen', but when the image is 

 compared with that of the objective of wider aperture (*65 N.A.), 

 the former image is found to be dim and ill-defined. The muscular 

 sheath is so ill-defined that it would not be noticed at all if it had 

 not been clearly revealed by the objective of wider aperture. But, 

 011 the other hand, the objective of greater aperture not only shows - 

 the muscular sheath, but it also shows the elongated nuclei of the 

 muscle cells ; and at the same time brings out the convoluted vessels 

 lying in the muscular sheath a plainly as if it were an object of 

 sufficient dimensions to lie upon the table appealing to the unaided 

 eye. 



We have pointed out in the proper place, 1 that although ' pene- 

 trating power ' varies inversely as the numerical aperture, it also 

 varies inversely as the square of the power. 



Now, from what we know of histological teaching in this country, 

 \vc do not hesitate to say that a histologist would not have attempted 

 to examine the above object with even a Zeiss A objective. He 

 would have advised the use of ' the J-inch,' of, perhaps, '65 aperture ; 

 but by so doing he would have secured only one-third of the pene- 

 trating power qua aperture, and one-seventh of the penetrating power 

 qua power. 



It is manifest, then, that pursuing this course in the histological 

 laboratory defeats the end sought, and which it is so desirable to 

 attain. 



It is absolutely unwise to use a higher power than is needful. 

 A j-inch where a Viiich would answer involves loss in many ways, 

 and would never be resorted to if the aperture of the lenses employed 

 n-ere as great as the power used legitimately permitted? 



A given structure, to be seen at all, must have a given aperture ; 

 to obtain this, as objectives now made for laboratory purposes run, 

 they are obliged to use too high a power. The result is that in seek- 

 ing to avoid what is accounted the loss of ' penetrating power ' at 

 an inverse ratio to the aperture, it is forgotten that we are losing it 

 inversely as the square of the power ! 



Moreover, the two apochromatic objectives we have already 

 referred to as test lenses are equally able to show the value of 

 apochromatism, not so much on account of the removal of the 

 secondary spectrum as for the reduction of the aberrations depend- 

 ent on the irrationality of the spectrum in ordinary achromatics. 



Use the 12 mm. '65 N.A. objective. Place a diatom in balsam 

 in the focus of it on a dark ground ; the diatom will shine with a 

 silvery whiteness, and the image will be wholly free from fog. 



Xow take one of the best achromatics obtainable of Vinch 

 focus of 80 (almost certainly a ^ in power) and examine the same 

 diatom in the same circumstances ; it will be bathed- in fog. If, 

 however, the achromatic objective is an exceptionally good one, and 

 we reduce its aperture to 60, we shall get a fair picture of the 

 1 Chapter I. 2 Chapter II. 



