OBJECTIVES AND OCULARS. 
59 
linger’s “ Carpenter/’ “ If a transverse section of an 
object is magnified ioo times in breadth, the distance 
between the planes of parts lying one behind the other 
is magnified 10,000 times at the corresponding parts on 
the axis when the object is in air, 7,500 times when it 
is in water, and 6,600 times when it is in Canada 
balsam.” It is, therefore, evident that the best results 
will be obtained by using as low a power and as high 
an angle as possible. 
This combination of wide aperture and low power, 
together with good corrections of spherical and other 
aberrations, is found in the apochromatic lenses in the 
greatest degree, the semi-apochromatics coming next 
in tins respect. Practice as well as theory proves this 
position : for it will be found that much better results 
will be got in observation, as in photo-micrography, by 
using a magnification as low as consistent with the 
eye’s power of vision, and an aperture as high as con¬ 
sistent with perfection in optical mechanism. 
This remark applies as much to physiological and 
histological research as to any other branch, though 
physiologists seem to be the slowest to recognise the 
truth. To separate or “ resolve ” details of a given 
fineness, or at a given distance apart, requires a given 
aperture ; if a lens has not sufficient aperture to effect 
the resolution, no amount of magnification will cause 
that lens to show us the details clearly separated. 
And every advantage lies with definition as against 
mere magnification; for with a low magnification, pro¬ 
vided it is enough to make the details large enough for 
