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The denominator sin a is the quantity well known (after Abbe) as the " numerical aperture."
Generally, to find the sum or difference of two or more fractional numbers, we must replace them by other fractional numbers having the same denominator; it is usually most convenient to take as this denominator the L.C.M.
can be applied to n, n', the denominator n remaining unaltered.
A fractional number is called a proper fraction or an improper fraction according as the numerator is or is not 3 less than the denominator; and an expression 4 such as 24 is called a mixed number.