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For average photography, the camera fitted with Kodak Ball Bearing shutter and tested meniscus achromatic lens is amply efficient, for it will make excellent pictures under ordinary conditions.

A similar argument may be applied to find at what point an achromatic lens becomes sensibly superior to a single one.

Photographic Query.—Given the diameter and focal length of a simple achromatic lens; at what distance from it must a diaphragm of given diameter be placed to give the best possible image?

If the web of a frog's foot be placed on the stage of a microscope and examined with an achromatic lens, the chromatophores can readily be made out.

The two spectra do not coincide, and the result, if an achromatic lens be made of these glasses, must be a remnant of color which cannot be neutralized.