# Page:Light waves and their uses.djvu/47

In the case of a telescope the corresponding limiting angle is the angle subtended by r at the distance F, i. e., rF, and this, by the formula, is the same as the angle subtended by the light wave at the distance D—the diameter of the objective. This limiting angle for a five-inch lens would, therefore, be ${\displaystyle \scriptstyle {\frac {1}{250000}}}$ of an inch, i. e., about the size of a quarter of a dollar viewed at the distance of a mile. This could be measured to within one-fifth of its value, so that the accuracy of measurement in this case corresponds to ${\displaystyle \scriptstyle {\frac {1}{12500000}}}$ as against ${\displaystyle \scriptstyle {\frac {1}{2500}}}$ without the lens; i. e., the order of accuracy is increased about five hundred times.