122 THE AMERICAN MONTHLY [Jun 
requisitioned to reflect the light into the condenser. Any 
light source which does not lend itself to being sharply 
focussed on the object, such as the incandescent electric 
lamp should be avoided. (6) That the aperture of the sub- 
stage condenser diaphragm be, at least, large enough to 
illuminate three-quarters of the diameter of the back lens 
of the objective employed. (c) That the microscope tube 
length be such as to exactly compensate for variations in 
the thickness of the cover glass, this correction being 
readily effected by pulling out or sliding in the draw- 
tube, and by noting the varying clearness of the image. 
The best point is easily determinable when a large il- 
luminating cone is used. | 
As to the instrument and indispensable accessories, it 
need only be said that our opticians now construct ad- 
mirable small microscope stands, provided with good fine 
adjustments, at a cost of from $20 to $30 ; that the semi- 
apochromatic Leitz objectives are both cheap and most. 
satisfactory in their performance when used in conjune- 
tion with very large illuminating cones, and that an ap- 
lanatic substage condenser is procurable at a trifling out- 
lay. With such an outfit, costing in all not more than 
$45, results of an exceedingly high order, with low and 
medium magnifications, are readily attainable, the most 
massive and rigid stands of the Powell No. 1 type, and 
apochromatic objectives, being only absolutely neces- 
sary in the most difficult branches of research, and when 
the best attainable photomicrographs are in question. 
Now we come to the fourth point, and have to consider 
the opponents of large-cone critical images on theoreti- 
cal grounds, ¢. e., those who think with Abbe that central 
illumination with a narrow incident pencil, the perfectly 
logical corollary drawn by him from his diffraction theory 
of microscopical vision, is the necessary condition for the 
attainment of an image strictly similar to the object, or 
at least the nearest possible approach thereto capable of 
