14 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



lens being bordered on one side with a purple fringe, and on the 

 other with a green fringe. Moreover, such a lens is not corrected 

 for Spherical aberration; and it must of course be rendered free 

 from this to be of any real service, however complete may be the 

 freedom of its image from false colours. This double correction 

 may be accomplished theoretically by the combination of three 

 lenses, namely, a double-concave of flint placed between two 

 double-convex of crown, ground to certain curvatures ; and this 

 method has long been employed in the construction of the hrge 

 object-glasses of Telescopes, which are, by means of it, rendered 

 Achromatic, — that is, are enabled to exert their refractive power 

 without producing either Chromatic or Spherical aberration. 



13. It has only been in comparatively recent times, however, 

 that the construction of Achromatic object-glasses for Microscopes 

 has been considered practicable ; their extremely minute size 

 having been thought to forbid the attainment of that accuracy 

 which is necessary in the adjustment of the several curvatures, in 

 order that the errors of each of the separate lenses which enters 

 into the combination, may be effectually balanced by the opposite 

 errors of the rest. The first successful attempt was made in this 

 direction, in the year 1823, by MM. Selligues and Chevalier of 

 Paris ; the plan which they adopted being that of the combination 

 of two or more pairs of lenses, each pair consisting of a double- 

 convex of crown-glass, and a plano-concave of flint. In the next 



year, Mr. Tulley, of London, with- 

 out any knowledge of what had 

 been accomplished in Paris, ap- 

 plied himself (at the suggestion of 

 Dr. Goring) to the construction 

 of Achromatic object-glasses for the 

 Microscope ; and succeeded in pro- 

 ducing a single combination of three 

 lenses (on the telescopic plan), 

 the corrections of which were ex- 

 tremely complete. This combina- 

 tion, however, was not of high 

 power, nor of large angular aper- 

 ture ; and it was found that these 

 advantages could not be gained 

 without the addition of a second 

 combination. Prof. Amici at 

 Modena, also, who had attempted 

 the construction of microscopic 

 object-glasses as early as 1812, but, 

 despairing of success, had turned 

 his attention to the application 

 of the reflecting principle to the 

 Microscope, resumed his original labours on hearing of the success 



Section of an Achromatic 

 Object-glass, composed of three 

 pairs of lenses, 1, 2, 3, each 

 formed of a double-convex of 

 crown-glass and a plano-con- 

 vex of flint ; ab c, its Angle of 

 Aperture. 



