OPTICAL IMAGES. 
49. Several transparent substances having this important 
property are found among the precious stones, and more parti¬ 
cularly in the diamond, which has a greater refracting power 
than any known transparent body. 
This advantage, and some other optical properties, induced 
some scientific men, among whom Sir David Brewster held a 
conspicuous place, to cause lenses to be made of diamond, 
sapphire, ruby, and other precious stones, and sanguine hopes 
were entertained of vast improvements in microscopes, resulting 
from their substitution for glass lenses. These hopes have 
however proved delusive. 
50. Notwithstanding all that enterprise, skill, and perseverance 
could accomplish, both on the part of scientific men, such as 
Sir David Brewster, and practical opticians, such as Pritchard 
and Charles Chevalier, the attempt has been abandoned. Inde¬ 
pendently of the cost of the material, difficulties almost 
insuperable arose from the heterogeneous nature of the gems. 
Their double refraction, and the imperfect transparency and 
colour of some of them. The improvement of simple microscopes 
composed of glass lenses by the invention of doublets, and by the 
proper combination and adaptation of their curvatures, was also 
such as to render their performance little, if at all inferior even 
to the gem lenses, while their cost is not much more than 
a twentieth of that of the latter. v 
In all cases, therefore, where objects or parts of objects of 
extreme minuteness are submitted to microscopic examination, 
requiring the application of high magnifying powers combined 
with extreme precision of definition, the compound microscope 
must be resorted to. 
51. Although it is not possible to efface altogether the effects 
of spherical aberration, yet they have been so considerably 
diminished by the adaptation of the curvatures of the lenticular 
surfaces, that in well-constructed optical instruments they may 
be regarded as entirely removed for all practical purposes. This 
is accomplished by giving to the two sides of the lens different 
curvatures, so adapted that the aberration produced by one shall be 
more or less counteracted by the aberration produced by the other. 
It has resulted from a mathematical analysis of the phenomena, 
that the lens which has least spherical aberration is double 
convex with unequal convexities, the radius of the flatter side 
being six times that of the more convex side. If the object to 
which such a lens be presented be very distant from it, and con¬ 
sequently the image proportionately close to it, the more convex 
side should be presented to the object. This, for example, is the 
case in all forms of telescopes and opera-glasses. But if, as is 
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