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The May electric ophthalmoscope is the instrument most generally used. The light, derived from a lamp with a loop or a curled filament, is fixed on the end of a stem projecting from a battery handle. The tube of the head of the ophthalmoscope, within which this stem is adjustably mounted, holds a condensing lens and a reflecting prism. This lens and prism project the light from the lamp into the eye. Behind the reflecting mechanism is a lens disk provided with a series of lenses, concave and convex, which follow each other in regular order from weaker to stronger. Each lens can be placed in the sight opening, located above the level of the reflector, by manipulating the milled edge of the disk. Opposite each lens is an illuminated number, indicating its strength in diopters.