16 
THEORY OF THE MICROSCOPE. 
figure, is purposely produced, to balance the small amount 
of aberration which remains otherwise uncorrected in the nega- 
tive eye-piece. The aberration of sphericity connected with 
the object-glass, is slightly under-corrected, as is shown by 
the images B B and R B, which are turned with their concave 
sides towards the object-glass. 
28. Action of the Negative Eye-Piece. The field lens F 
F, bends the lateral pencils inwards, forming images nearer 
and smaller than would have been formed without its action. 
This action of the field-lens diminishes somewhat the magnify- 
ing power of the instrument, but it enlarges the field of view, 
and it is hence called the field-lens. 
Secondly : the field-lens refracting the blue-rays more strongly 
than the red, the blue and red images are brought nearer 
together, as shown at B' B', R' R'. 
Thirdly : the blue rays being bent out of their course by the 
field-lens more than the red, the blue image becomes so much 
smaller than the red that both images are seen in the same 
direction, as shown by the lines of sight, a (?, and a' O", Fig. 5, 
which, by their intersection at the optical centre of the eye- 
lens, form the visual angle under which the magnified image of 
the object is seen, as though situated at O O' O". 
Fourthly : the field-lens being thicker in its centre than at the 
edges, and the rays from the centre of the object all falling 
upon the central portion of the field-lens, while the pencils 
from the extremities of the object fall upon the borders of the 
field-lens, the curvature of the image is reversed, and the images 
B' B', R R', have their concave surfaces turned towards the 
eye-lens, and are in the exact condition to appear as a single 
straight image at O O' O", when viewed through the eye- 
lens E E. 
Fifthly: the lines a O, and a' 0'\ passing through the optical 
centre of the eye-lens, and through the extremities of the real 
images B' B', and R' R', if extended back to the limit of dis- 
tinct vision, (which is generally reckoned at ten inches,) show 
the direction and apparent size of the magnified image, as it 
appears to the eye. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
