1.It should have the highest possible aperture (lowest possible f number) for the efficient collection of light radiating from the source 2.It should be as powerful as possible in order to project the required filament image in the shortest possible distance. 3.It should have as large a diameter as possible to enable illumination of the large fields covered by low power objectives. 4.Its diaphragm should be close to the condenser field lens, and should be capable of closing down to a very small aperture — small enough to encroach upon the field of the highest power objective. 5.It should be well corrected for spherical aberration so that all light collected from the source is focused into a single image plane, and so that even a small source placed at its focus is capable of filling it evenly with light.