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. 



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 natter 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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