ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
ideal image A, reproducing all possible detail, which only light of 
infinitesimal wave-length would be competent to produce. 
B. Image B, the standard image. 
C. Image C, standard image No. 2, is the image which the light 
taken in by the objective would, if reversed, produce. 
D. Image D, the focal image formed by the objective, is an enlarge- 
ment with distortion of image C. 
E. Image E, the visual image , is the virtual image formed by the 
eyc-piece, an enlargement with slight distortion of image D. 
F. Finally, image F is the image actually produced on the retina 
of the observer. 
Fig. 2. 
The consideration of the transition from A to B leads to the conclu- 
sion that any speck in the image, when best seen, will have an apparent 
diameter of A/4 ; consequently the corresponding portion of the object 
(viz. a globe with diameter A/4) contains an enormous amount of detail, 
all of which is massed together in the standard image as one speck. 
The investigation of the other transitions (from B to C and so on) shows 
that it is upon image C, the standard image No. 2, that we must fall 
back to determine what is the detail in the image E and its true nature ; 
and that the adjustments of the Microscope must be made with a view to 
the improvement of this image C. 
The course of an individual beam from this standard image C (fig. 2) 
is then traced. The dark line in the figure represents the axial ray. 
The beam is brought to a focus at x , then diverges and helps to form the 
image D, and after passing through the eye-piece is brought to a second 
focus at y , where the eye-piece can form an image of x. The light of the 
beam advancing in the direction qy will enter the eye as if it had come 
direct from the whole extent of the image at E. After passing y , the 
beam again diverges, but is bent by the action of the front half of the 
eye so as to again intersect the optic axis of the Microscope where that 
axis pierces the retina of the observer ; and there it assists in the forma- 
