176 Mr Lister on the improvement of the 



form for the correcting flint lens, which was probably adopt- 

 ed at first for its simplicity, has in that quality a strong re- 

 commendation ; particularly as it obviates the danger of error 

 which otherwise exists in centering the two curves, and there- 

 by admits of correct workmanship for a shorter focus. To 

 cement together also the two lenses of the glass, diminishes by 

 very nearly half, the loss of light from reflection, which is con- 

 siderable at the numerous surfaces of a combination. I have 

 thought the clearness of the field and brightness of the picture 

 evidently increased by doing this ; it prevents any dewiness or 

 vegetation from forming on the inner surfaces ; and I see no 

 disadvantage to be anticipated from it, if they are of identical 

 curves and pressed closely together, and the cementing medi- 

 um permanently homogeneous. 



These two conditions then, that the flint lens shall be plano- 

 concave, and that it shall be joined by some cement to the con- 

 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 flint-glass, 

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

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

 portion has been such between the refractive 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 parallel. 



L a b, Fig. 4, be supposed such an object-glass, and let it be 

 roughly considered as a plano-convex lens, with a curve a cb 

 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 fdeg 

 issuing from the radiant point/,- 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 incidence f d i, being probably almost three times as 

 great. 



