22 
THEORY OF THE MICROSCOPE. 
should be reckoned, is at a considerable distance behind the 
exterior surface of the anterior lens. 
37. Foci of liigh powers iiicoiiveiiiently near to tlie 
Otojective. In improved triple achromatic objectives of high 
magnifying power and mry large angular aperture, the diame- 
ter is only sufficient to admit the proper pencil ; the convex 
lenses are wrought to an edge, and the concave lenses have 
only sufficient thickness to support their figure ; consequently 
the entire combination is the thinnest possible, and yet the 
focus of the compound achromatic objective is so near the 
anterior surface of the front lens that an object can only be 
covered with the thinnest film of glass which can be obtained, 
when it is viewed with the highest powers. 
J. k W. GRUNOW & GO’S ILLUSTRATED 
