Light visible and invisible : a series of lectures delivered at the Royal Institution of Great Britain, at Christmas, 1896 (1897)
Thompson, Silvanus Phillips, 1851-1916, Royal Institution of Great Britain · New York : Macmillan · leaf 79
he whole chord,or is the versed-sine of the semi-angle subtended at the centre. In Continental worksit is frequent to use the namesagitta for the length of this lineMA; and as this term is pre-ferable to versed-sine, and canbe used generally irrespective ofthe size of radius, it is hereadopted. The… … MB 2;--AM But, for small apertures, AM is small compared with 2r,and may be neglected in the denominator, whence, to afirst approximation, MA = -.- .2 r Twice1 the sagitta represents numerically the curvature.The error is less than one per cent when the semi-angle sub-tended at the centre is io° ;…
Light · X-rays
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