Focometer

Invention of Silbermann's for measuring the focal lengths of convex lenses. It is based on the principle that when the object and its image are equidistant from the lens, their distance from each other is four times the focal length. It consists of a graduated rod with three sliders, the middle one carrying the lens and the others carrying two thin plates of some translucent material, ruled with lines which are the same distance apart in both. The sliders must be adjusted till the image of one of these plates is thrown upon the other without change of size, as tested by the coincidence of the ruled IMG:content-0115.png:[blocks in formation] Fiber Matter Pasture Gra IMG:content-0116.jpg:[blocks in formation] IMG:content-0117.jpg:[blocks in formation] Corn IMG:content-0118.jpg:[blocks in formation] IMG:content-0119.png:[blocks in formation]