226 THE MICROSCOPICAL NEWS. 
dressed, and let this be the standard for comparison. The real 
difficulty in such microscopical investigation is in getting a pure 
standard sample ; but with this absolutely secured, all difficulties 
vanish, and every microscopist should be able to detect adulteration 
easily and quickly. 
THE RELATION OF APERTURE AND POWER IN 
THE MICROSCOPE. f 
By ProFressor E. AxpBE, Hon. F.R.M.S. 
(Read 14th June, 1882). 
Ll.—The Rational Balance of Aperture and Power. 
(t2.) Division of the Entire Process of the Microscope between 
Ocular and Objective. 
(Continued from page 213.) 
HE sources of the residuary aberrations in question are perfectly 
well known in theory. Some of them result from the dispro- 
portionate increase of the positive and negative spherical aberra- 
tions in different parts of the system, arising from the increase of 
obliquity and in regard to different colours, which disproportionality 
prevents a strict compensation of the opposite spherical aberrations 
even for the rays of one colour and gives rise to still more con- 
siderable residuals in the totality of the rays of mixed light. 
Other defects arise from the disproportionate increase of the 
dispersion from the red to the blue, which is found in all kinds of 
optical glass hitherto produced, and forbids a really perfect chro- 
matic correction of the systems. In regard to all of these 
aberrations it may be readily shown that they must introduce 
greater and greater uncorrected residuals as the numerical aperture 
of the cone of collected rays is more and more increased, other 
circumstances being equal.* 
It is therefore obvious that under equal conditions of technical 
construction the inherent dissipation of the light will always be 
greater with the wider apertures, and consequently the super- 
amplification which is compatible with any given degree of 
precision of the image will be confined to a dower figure with 
* That the numerical aperture is the essential element and of the aperture- 
angle, results from the fact that all the effects considered above depend on the 
proportion of the clear aperture to the focal length of the objective, which pro- 
portion is exactly expressed by the numerical aperture. 
