The balancing of the series of forces will not be complete until the respective **triplets** of conjunctions have filled up the entire space between them.

**Triplets** are employed when the focal length of the simple microscope was less than in.

**Triplets**, quadruplets, &c., be made with n symbols, so that (1) every contiguous pair of symbols in one multiplet are a contiguous pair in some other, the first and last of any multiplet being considered contiguous, and (2) no three symbols in any multiplet shall occur in any other.

If a is the distance shown by the normal **triplets** the type of separation observed in the line D2 shows distances from the central line equal to a /3, 3a/3, 5a/3, while the type of D 1 gives 2a/3, 4a/3.

Hamilton, still keeping prominently before him as his great object the invention of a method applicable to space of three dimensions, proceeded to study the properties of **triplets** of the form x+iy+jz, by which he proposed to represent the directed line in space whose projections on the co-ordinate axes are x, y, z.

Strontium also gives **triplets**, but only the side branch series has been observed.