28 PRACTICAL PHOTO-MICROGRAPHY. 



focus exactly in one plane. This supplementary lens is 

 usually a double convex fixed at the back of the objective- 

 cell ; it introduces extra reflecting surfaces which should be 

 avoided, and it sensibly alters the focal length of the objective. 

 By experiment the distance between the foci of any lens may 

 be determined and allowance made before each exposure by 

 withdrawing the sensitive plate from the objective to the 

 amount determined by the experiments ; but as the distance 

 differs not only for each objective but also for each distance 

 from object at which the objective is used, it is plain that 

 such a necessity would be an intolerable infliction to most 

 men. Many, if not all, opticians now produce objectives of 

 all powers most accurately corrected for the actinic rays, and 

 there is no extra cost involved. The new apochromatic object 

 glasses used with projection oculars are perfect in this respect, 

 and have so many other valuable qualities that we propose to 

 devote a chapter, or at least a paragraph, to them solely. 



A much vexed question is that of the angular apertures of 

 objectives not only for photo-micrographic purposes but for 

 general purposes of observation. " Resolution," or the power 

 of separating visibly line from line, dot from dot, mark from 

 mark, increases with angular aperture. Thus an objective 

 having an angular aperture of 20 deg. will, with blue light, 

 visibly separate lines about 18,000 to the inch, while a lens of 

 120 deg. will, under similar conditions, separate lines about 

 90,000 to the inch. (For further remarks on angular aperture, 

 immersion, and numerical aperture, see Chapter XXIY, 

 pages 162 et ssq.) But as aperture increases certain useful 

 qualities fall off. A quality called " penetration " is known to 

 fall off in proportion as aperture increases, and this matter re- 

 quires investigation. " Penetration," as it is called in micro- 

 scopy, " Depth of Focus," as it is called in photography, is a 

 supposititious power of focusing, sufficiently sharply to prevent 

 visible blur, simultaneously on several planes perpendicular to 

 the optical axis of the system. Plainly the essence of the debate 

 lies in the amount of blur visible or permissible. In a small 

 photograph a very small area of confusion is permissible, while 

 in a larger photograph a comparatively large area of confusion 



