30 PRACTICAL PHOTO-MICROGRAPHY. 



tration there is another matter to be considered. While it is 

 true that penetration decreases in direct proportion as angular 

 aperture increases, it is also true that penetration decreases 

 much more rapidly as magnification increases. Penetration 

 varies inversely as aperture but also inversely as the square 

 of magnification. So that a low power with a wide angular 

 aperture may be expected to yield a better result in the matter 

 of penetration than a higher power mth an equal aperture. 

 As, therefore, aperture is the means whereby we gain resolu- 

 tion and definition, and as magnification can be obtained in 

 other ways than by the use of a high power objective, the ad- 

 vantage clearly lies with the use of a low power of wide angle, 

 magnification being obtained by stretch of camera, eye- 

 piecing, or " camera enlargement " of the original negative. 

 The limit to the angular aperture of a glass in proportion to 

 its focal length is a difficulty of optical mechanism. Beyond 

 a certain point angular aperture in high proportion to focal 

 length cannot be achieved by practical opticians. 



There is, however, one defect inseparable from the use of 

 very wide angled objectives, and as it was noticed by Dr. Car- 

 penter in his great book, "The Microscope and its Revelations " 

 (London : Churchill. Sixth edition, 1881), it may well be put 

 in his own words. After dwelling upon the difficulty of per- 

 fectly correcting a wide-angled lens for spherical and chromatic 

 aberrations, and after pointing out the advantages in this 

 respect gained by the system of homogeneous immersion, he 

 proceeds thus : " But here comes in another source of impair- 

 ment the difference in the perspective views of every object 

 not a mere mathematical point or line which are received 

 through the different parts of the area of the objective" Dr. 

 Carpenter then quotes in support of his position such high 

 authorities as Dr. Royston Piggott, and Messrs. Dallinger and 

 Drysdale. We might admit this defect more readily if we 

 were certain that " perspective" can exist in a diffraction 

 image, but we still believe that even if Dr. Carpenter's view 

 be correct, better results on the whole will be obtained by the 

 use of as wide angles as can be used without serious amounts 

 of aberration. 



