64 Modern Microscopy 



Numerical Aperture and Power. — As we have before 

 remarked, magnifying power is not the only quaUty neces- 

 sary for the observation of minute structure. The power to 

 deUneate fine detail is still more dependent on the numerical 

 aperture of the objective. It has been explained by Dr. 

 Dallinger in ' Carpenter on the Microscope,' and will be 

 evident from a consideration of the preceding remarks con- 

 cerning numerical aperture, that two objectives — one of 

 much greater magnifying power than the other, but both 

 having only the same numerical aperture — will only divide 

 the same amount of detail, the higher power only exhi- 

 biting it on a larger scale. That is, supposing with a 

 ^-inch objective of 0'90 N.A. certain structure were pre- 

 sented, and then a ^-inch objective with just double the 

 magnification, but with the same N.A., were afterwards 

 used, there would be no further power of resolution in the 

 ^ than in the j. It might be possible to make an objective 

 of very low power of sufficiently high aperture to divide 

 very minute details, but this would be useless unless the 

 objective would bear a sufficiently deep eyepiece to enable 

 the human eye to see it. It therefore becomes necessary 

 that a ratio of aperture to power should be established. 

 Mr. Nelson has suggested that a standard, termed the 

 * optical index ' (O.I.), should be adopted for this purpose, 

 to indicate the numerical aperture that should be given to 

 an objective, if it be intended that the eye should see in the 

 image as fine detail as it could divide in a real object of the 

 same size. It is ascertained by multiplying the numerical 

 aperture of an objective by 1,000, and dividing by the initial 

 magnifying power of the objective.* If a microscope is 

 required to show all that keen eyes are able to appreciate, 

 then 0"26 N.A. must be given to it for every 100 diameters 

 of magnification. If we limit the power of the eyepiece 

 of such a microscope to 10, then the objective must 

 have 0'26 N.A. for each 10 diameters of initial magnifying 



E. M. Nelson, Journal of the Royal Microscopical Society, February, 



