PRACTICAL PHOTOMICROGRAPHY. 29 



is not only permissible, but preferable to over-sharpness 

 from an artistic point of mew. In photo-micrography as a 

 science we have nothing to do with fine art, and scientifically 

 speaking' no blur or confusion-area is permissible at all. And 

 further a lens cannot by any possibility focus equally sharply 

 on any two planes perpendicular to its axis, and any appear- 

 ance of equal sharpness can only be attained by a general sac- 

 rifice of sharpness, o by a compromise between absolute 

 sharpness on one plane and absolute sharpness on another 

 plane. When such a compromise is made the image may ap- 

 pear sharper generally, but that is simply because there being 

 no absolute sharpness anywhere there is a lack of sharp to 

 compare with un sharp, and so the eye is deceived into an im- 

 agination of sharpness. And yet again it is not the case that 

 a compromising or " diffusing " lens gives the sharpest image 

 as a whole, for a lens capable of giving the utmost definition 

 on any one plane will certainly show adjacent planes propor- 

 tionately sharper than the lens which is incapable of giving 

 thorough definition on any plane. It is our belief that this 

 misleading theory of penetration, promulgated and upheld by 

 great authorities mistaken on this point, has done much mis- 

 chief to microscopic optics, and led many an optician and 

 many a worker astray. For purposes of rapid observation of 

 moving objects, where general appearances are desired rather 

 than critical examination, undoubtedly low angle " compromis- 

 ing " lenses are of the greatest service, but the writer ventures 

 to assert, both as a theory and from careful and repeated ex- 

 periment, that better photo-micrographs are produced, and pre- 

 sumably better images observed, by well-made, wide-angle 

 lenses than by lenses made for " penetration," or stopped down 

 so as to cut off available angle. Certainly many lenses are 

 made so imperfectly that when they are used at their full 

 available angle of aperture aberrations come in that spoil their 

 performance altogether, but this is a mechanical not a theo- 

 retical fault. The writer regrets to say that all this is in flat 

 contradiction to what he wrote with reprehensible precipit- 

 ancy when he was but a beginner and a "smatterer" in this 

 science. But granting the desirability of this quality of pene- 



