THE MICROSCOPE. 



26 



vex, seem desirable to be taken as a basis for the microscopic 

 object-glass, provided they can be reconciled with the destruc- 

 tion of the spherical and chromatic aberrations of a large 

 pencil. 



* ' Now in every such glass that has been tried by me which 

 has had its correcting lens of either Swiss or English glass, 

 with a double convex of plate, and has been made achromatic 

 by the form given to the outer curve of the convex, the pro- 

 portion has been such between the refrac- 

 CJ H tive and dispersive powers of its lenses, that 

 its figure has been correct for rays issuing 

 from some point in its axis not far from its 

 principal focus on its plane side, and either 

 tending to a conjugate focus within the tube 

 of a microscope, or emerging nearly par- 

 allel. 



" Let A B (Fig. 13) be supposed such an 

 object-glass, and let it be roughly considered 

 as a plano-convex lens, with a curve A G B 

 running through it, at which the spherical 

 and chromatic errors are corrected which are 

 generated at the two outer surfaces; and let 

 the glass be thus free from aberration for 

 rays F D E G issuing from the radiant 

 point F, H E being a perpendicular to the 

 convex surface, and I D to the plane one. 

 Under these circumstances, the angle of 

 emergence G E H much exceeds that of in- 

 cidence F D I, being probably nearly three 

 times as great. 



"If the radiant is now made to approach 

 the glass, so that the course of the ray F D 

 E G shall be more divergent from the axis, as the angles of in- 

 cidence and emergence become more nearly equal to each 

 other, the spherical aberration produced by the two will be 

 found to bear a less proportion to the opposing error of the 

 single correcting curve A C B; for such a focus therefore the 

 rays will be over-corrected. 



"But if F still approaches the glass, the angle of incidence 



Fig. 13. 



