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LENSES AND MIRRORS 123
surface. Focus the distant object as sharply as possible on the paper screen, while an assistant measures the distance from the flat side of the lens to the scn-en. Reverse the lens, holding its convex side toward the screen and the flat side parallel with the screen's surface. Again Eocus the object sharply and measure the distance from the flat side of the lens to the screen. Add these two measurements and divide by 2. The result will be a close approximation of the focal length of the lens.
(105) Bi-convex lenses may he measured the same way. Focus a distant object sharply on the paper screen and measure from the center of the lens edge to the screen surface. That will be the focal length of the lens, provided the lens has been held parallel with the screen surface.
Focal Length of Simple Lenses
(106) Be sure the lens is perfectly clean. Hold it parallel with the screen so that its edge all around is equi-distant from the screen surface. Focus an object no less than 50 feet away. An object 100 feet or more away is better since it allows the light to enter the lens in essentially parallel rays which leads to more accurate conclusions. The room should be as dark as practicable when making the measurement, because too much light makes it difficult to judge the sharpness of focus.
Measuring Working Distance
(107) To measure a working distance, first focus the picture as sharply as possible on the screen. Stop the projector, remove the film and measure the exact distance from face of the tracks upon which the film rests in passing through the gate, to the first surface of the lens. If the uncoated side of the film is next to the aperture the thickness of the film must be added for exact accuracy. This measurement is the working distance often wrongly referred to as "back focus."
Mkasuring Equivalent Focus .
(108) This is seldom necessary but easy to do. Use two sheets of black or dark colored paper a few inches