74 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



crown-glass, that a convex Iqqs of the former, the focal length of 

 which is 7f inches, will prodnce the same degree of color as a 

 convex lens of crown-glass, whose focal length is 4J inches. 

 Hence a concave lens of the former material and curvature, will 

 fully correct the dispersion of a convex lens of the latter; whilst 

 it diminishes its refractive power only to such an extent as to 

 make its focus 10 inches. The correction for chromiatic aberra^ 

 tion in such a lens would be perfect, if it were not that, although 

 the extreme rays, violet and red, are thus brought to the same 

 focus, the dispersion of the rest is not equally compensated ; so 

 that what is termed a secondary spectrum is produced, the images 

 of objects seen through such a lens being bordered on one side 

 with a purple fringe, aud 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 tliis, to be of any 

 real service, however complete may be the freedom of its image 

 from false colors. The 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 large object-glasses of Tele- 

 scopes, 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 of late years, however, that the construc- 

 tion of Achromatic object-glasses for ^Microscopes has been con- 

 sidered practicable; their extremely minute size having been 

 thought to forbid the attainment of that accuracy which is neces- 

 sary in the adjustment of the several cui'vatures, in order that 

 the errors of each separate lens which enters into the combina- 

 tion, 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 M. Selligues of Paris ; the plan which he 

 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 plauo-concave of flint. In the next year, Mr. Tulley of 

 London, without any knowledge of what M. Selligues had ac- 

 complished, applied himself (at the suggestion of Dr. Goring) to 

 the construction of achromatic object-glasses for the microscope; 

 and succeeded in producing a single combination of three lenses 

 (on the telescope plan), the corrections of which were extremely 

 complete. This combination, however, was not of high power, 

 nor of large angular aperture ; and it was found that these ad- 

 vantages could not be gained, without the addition of a second 

 combination. Prof Amici at Modena, also, who 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 

 labors on hearing of the success of M. Selligues ; and, by work- 



