OPTICAL PRINCIPLES OF THE MICROSCOPE. 11 



lengthening the focus of the objective. In the Microscope, on the other 

 hand, the conditions are altogether different. For the object-glass 

 receives rays which diverge very widely from a near object, and the size 

 of the image formed by their convergence depends upon the propor- 

 tionate distances of the object and the image from the lens ( 8); mag- 

 nifying power being thus gained by shortening the focus of the object- 

 glass. And the chromatic and spherical aberrations resulting from the 

 incidence of diverging rays can only be fairly corrected by a single- 

 triplet combination, when its focus is long (giving a low magnifying 

 power), and the divergence of those rays moderate, so that the angle of 

 the aperture is small. 



15. It has only been in comparatively recent times that the construc- 

 tion of Achromatic object-glasses for Microscopes has been found prac- 

 ticable; 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 



PIG. 11. 

 PIG. 10. 



Section of an Achromatic Ob- 

 ject-glass, composed of three 

 pairs of lenses, 1, 2, 3, each form- 

 ed of a double-convex of crown- 

 glass and a plano-concave of flint ; 

 a b c, its Angle of Aperture. 



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

 the plan which they adopted being 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 following year, Mr. Tulley, of London, 

 without any knowledge of what had been accomplished in Paris, applied 

 himself (at the suggestion of Dr. Goring) to the construction of Achro- 

 matic object-glasses for the Microscope; and succeeded in producing a 

 single combination of three lenses, on the telescopic 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 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, despair- 

 ing of success, had turned his attention to the application of the reflect- 



