86 



PHOTO-MICROGRAPHY 



optical causes of this well-known fact ; indeed, it would be foreign to any article on 

 photo-micrography, but it serves to illustrate what N. A. really is to the microscopical 

 objective. The higher it is the more can be seen ; and the less you have, the more 

 objects seem to blend themselves into a state of indivisibility. Opticians, therefore, 

 have striven to obtain the highest degree of N. A. possible for each objective. The 

 higher the magnifying power, the more easily is this obtained, and we regret that 

 so many opticians understate the magnifying power of their lenses, so as to gain a 

 repute for making wide-angle objectives. For example, it is fairly easy to make a 

 ^th. of moderate N. A., and sell it for a Jth of high N. A. It is a fraud, but it is 

 often met with. With apochromatics manufactured by Zeiss, and Powell and 

 Lealand, the foci given by the makers may always, without exception, be regarded as 

 truthfully exact. The consequence of this is that the value of the magnifying power 

 of any of their objectives is readily known, and this is of no small service in ascertain- 

 ing how much the photo-micrographer has magnified his object ; but to this we shall 

 refer again when a suitable occasion arises. A practical point the photo-micrographer 

 should bear in mind is to always obtain, speaking generally, an objective with as high a 

 numerical aperture as he can, for a ^th which magnifies 60 diameters with a low N. A. 

 is not comparable in its powers of resolution to a ^th of high N. A., although it 

 amplifies 20 diameters less. Magnification, therefore, is not so important — it can be 

 got by camera-length or eye-piecing — as numerical aperture. In Figs, i o and 1 1 , 

 Plate III. are two pictures, one taken with a lens of low N. A., and the other with 

 an objective of high angle, the magnification being the same in both cases. One 

 shows the dots plainly, the other hardly at all. 



But from what has been said it would appear that low-angled objectifies are of no 

 use whatever. This is not the case. Seeing that although the depth of focus — 

 which means the power possessed by the objective of viewing simultaneously several 

 planes of focus — varies inversely as the square of the power, still it also varies as the 

 reciprocal of the N. A. ; so that the less the N. A. the more the depth of focus 

 ohtainable. 



Depth of focus used to be thought to be a distinct property peculiar to the best 

 objectives, but now that the philosophy of microscopical optics is better understood, it 

 is recognised not to have a separate existence of its own devoid of explanation, for it 

 is known to directly follow as the result of lowering the N. A. As therefore the 

 highest resolving power rests with lenses of the highest N. A., so the converse equally 

 holds true that the less the aperture the less the objective can resolve, but the greater 



