ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 785 
2. 
For two oblique 
cones (£' = 
163*22): 
h 
Co 
0"d 
0-0-55 
0- E 
0"f 
0*1 
+ 7* 3/a 
+ 4-9/a 
4 3 * 6/x 
4- 2*9/4 
4 2*1/4 
0-3 
— f-20 * 9 
4-13*8 
4 9*8 
4 7*9 
4 5*7 
0*5 
+32-3 
4-20*8 
414*4 
411-3 
4 7*9 
0-7 
4-40*2 
4-24*6 
+ 16*0 
4-12*0 
4 7*9 
0-9 
4-44*0 
424*8 
414*6 
4 9*9 
4 5*7 
1-1 
4-43*6 
-j-21*4 
410-1 
4 5*1 
4 1-8 
1*3 
4-39*7 
414*6 
4 2*8 
- 1*7 
- 2*8 
-1-3 
-39*7 
-14*6 
- 2*8 
4 1*7 
4 2*8 
-1*5 
-31*6 
- 6*2 
4 5*0 
4 8*6 
4 6*0 
-1*7 
-23*3 
4 2*1 
411-7 
413*5 
4 5*1 
-1-9 
-15*9 
4 8*2 
4-14*9 
413*8 
- 2*6 
-2*1 
-10*9 
410*1 
412*4 
4 7*5 
-19*5 
-2-3 
- 6*9 
4 9*2 
4 5*2 
- 4*8 
-45*4 
-2-5 
- 9*0 
- 0*2 
-12*4 
-28*9 
-86*7 
The radius of the circle of least confusion (87 /x for F) is thus not 
greater than for telescope objectives of great focal length. Besides, 
with Microscope systems, the marginal zone is best used for resolution, 
i. e. f >r the representation of striae, and for fine structures limitation of 
the illuminating cone (with change of adjustment) becomes necessary. 
Further, the above table shows that for the yellow-green rays, with 
central illumination the maximum of the transverse aberration, and with 
the combined effect of all zones the two maxima, deviate somewhat from 
the positive limit. Thus, by change of the spherical correction (of B! 
in equation 1) and £', a further diminution of the spherical circle of 
least confusion can be obtained. On the other hand, the incomplete 
coincidence of the yellow and blue circles of least confusion is not to be 
removed by change of the chromatic correction (of A 2 in equation 1), 
because by lessening this error, for example, for direct light, it only 
becomes worse for the other forms of illumination. In order to obtain 
a distinct image in the focal plane, the rays with strong diffusion, which 
only form the ground on which the real image is seen, are completely 
cut off. These are 
(1) All red rays since they are spread over ten times as large a 
surface as the image proper. 
(2) The blue rays at distance 2*1 to 2*5 mm. 
(3) The bright rays with great transverse aberration, which increases 
or diminishes rapidly and so produces a strong dispersion. These are 
the yellow rays of height of emergence 1*3 to 1*5 mm. and the green 
marginal rays. 
On the other hand, the rays which take part in the production of the 
image are : — 
(1) The weak rays with slight transverse aberration, which increases 
very slowly, so that there is a strong concentration of light. These are 
the blue rays at a distance of 0 to 2 mm. 
(2) The bright rays with great but slowly increasing aberration, e.g. 
the yellow-green rays of height of emergence 1*7 to 2*1 mm. 
(3) The weak rays with small transverse aberration, even if this 
increases rapidly, e. g. the yellow marginal rays. 
