PRACTICAL PHOTO-MICROGRAPHY. 163 



than the limit available with dry lenses, and the " fan " of 

 diffraction rays is by immersion " closed up." 



To narrate the causes and considerations that led to the 

 adoption of numerical aperture as the basis of calculation of 

 the apertures of objectives would be both tedious and out of 

 place. We content ourselves, therefore, with saying that to 

 Professor Abbe is due the system of calculation now almost 

 universal, and by permission of the Royal Microscopical 

 Society, we give in the latter part of this book a table of 

 apertures, showing the relation between " N. A." and angular 

 aperture, together with other matter of great value to the 

 microscopist. It is important to remember that resolution is 

 proportional to numerical aperture but not to angular aper- 

 ture. 



The maximum air angle being 1, we have oil immersion 

 lenses with numerical aperture as high as 1.5, but nothing 

 above 1.43 has, so far as we know, been made practically useful. 



The immersion system is also used for condensers, and 

 theoretically we ought to be able to use the whole aperture of 

 the highest apertured glasses, but few, if any, glasses are 

 sufficiently well corrected to stand utilization of their entire 

 aperture. (See paper by Mr. E. M. Nelson in "English 

 Mechanic," No. 1,234, Nov. 1888, for information on this 

 subject,) Messrs. Powell and Leland make a fine apochromatic 

 condenser N. A. 1.4. Zeiss constructs a similar article N. A. 1, 

 while many opticians make non-achromatised oil-immersion 

 condensers up to N. A. 1.4, or nearly so. 



Apochromatic homogeneous immersion objectives of high N. 

 A. are at present the acme of microscopical practical optics. 

 But while resolving power increases with numerical aperture 

 the quality called penetration decreases as resolving power 

 increases. We have, however, tried to show in an earlier 

 chapter that this " penetration " is a bogus quality, and, in fact, 

 a defect though a deceptive one. Nevertheless there are 

 occasions not a few when moderate sharpness on various planes 

 is preferable to absolute sharpness on any one plane, and in 

 such cases the utilized aperture of the lens may be easily cut 

 down by stopping down the condenser to any desired extent 



