[Proceedings of the Royal Irish Academy, Nov. 13, 1843,
vol. 2, 424-434]
It is known to all students of algebra that an imaginary equation of
the form 
has been employed so as to conduct to very varied
and important results. Sir Wm. Hamilton proposes to consider some of
the consequences which result from the following system of imaginary
equations, or equations between a system of three different
imaginary quantities:
no linear relation between i, j, k being supposed to exist, so that the equation
in which
and w, x, y, z, w', x', y', z' are real, is equivalent to the four separate equations
Sir W. Hamilton calls an expression of the form 
a
quaternion; and the four real quantities w, x, y, z he
calls the constituents thereof. Quaternions are added or
subtracted by adding or subtracting their constituents, so that
Their multiplication is, in virtue of the definitions (A) (B) (C), effected by the formulae
which give
and therefore
if we call the positive quantity
the modulus of the quaternion . The modulus of the
product of any two quaternions is therefore equal to the product of
the moduli. Let
then, because the equations (D) give
we have
Consider x, y, z as the rectangular coordinates of a point of
space, and let 
be the point where the radius vector of
x, y, z (prolonged if necessary) intersects the spheric surface
described about the origin with a radius equal to unity; call
the representative point of the
quaternion , and let the polar coordinates 
and 
,
which determine upon the sphere, be called the
co-latitude and the longitude of the representative
point , or of the quaternion itself; let also the
other angle 
be called the amplitude of the
quaternion; so that a quaternion is completely determined by its
modulus, amplitude, co-latitude and longitude. Construct the
representative points 
and 
, of the other
factor 
, and of the product 
; and complete the
spherical triangle 
by drawing the arcs
, 
, 
. Then, the
equations (G) become
and consequently shew that the angles of the triangle
are
these angles are therefore respectively equal to the amplitudes of the
factors, and the supplement (to two right angles) of the amplitude of
the product. The equations (D) show, further, that the
product-point is to the right or left of the
multiplicand-point , with respect to the
multipler-point , according as the semiaxis of +z
(or its intersection with the spheric surface) is to the right or left
of the semiaxis of +y, with respect to the semiaxis of +x: that
is, according as the positive direction of rotation in longitude is
towards the right or left. A change in the order of the two
quaternion-factors would throw the product-point from the
right to the left, or from the left to the right of .
It results from these principles, that if
be any spherical triangle; if, also,
be the rectangular coordinates of ,
those of , and
of ,
the centre of the sphere being origin, and the radius being unity; and
if the rotation round +x from +y to +z be of the same
(right-handed or left-handed) character as that round from
to ; then the following formula of
multiplication, according to the rules of quaternions, will hold good:
Developing and decomposing this imaginary or symbolic formula (I), we find that it is equivalent to the system of the four following real equations, or equations between real quantities:
Of these equations (K), the first is only an expression of the
well-known theorem, already employed in these remarks, which serves to
connect a side of any spherical triangle with the three angles
thereof. The three other equations (K) are an expression of another
theorem (which possibly is new), namely that a force
, directed from the centre of the sphere to the
point , is statically equivalent to the system of three
other forces, one directed to , and equal to
, another directed to , and
equal to 
, and the third equal to
, and directed
towards that pole of the arc , which lies at the same
side of this arc as . It is not difficult to prove this
theorem otherwise; but it may be regarded as interesting to see that
the four equations (K) are included so simply in the one formula (I)
of multiplication of quaternions, and are obtained so easily by
developing and decomposing that formula, according to the fundamental
definitions (A) (B) (C). A new sort of algorithm, or calculus, for
spherical trigonometry, appears to be thus given, or indicated. And
by supposing the three corners of the spherical triangle
to tend indefinitely to close up in that
one point which is the intersection of the spheric surface with the
positive semiaxis of x, each coordinate 
will tend to become
= 1, and each 
and 
to vanish, while the sum of the
three angles will tend to become 
; so that the following well
known and important equations in the usual calculus of imaginaries, as
connected with plane trigonometry, namely,
(in which ), is found to result, as a limiting case, from
the more general formula (I).
In the ordinary theory there are only two different square roots of negative unity (+i and -i), and they differ only in their signs. In the present theory, in order that a quaternion, w + ix + jy + kz, should have its square = -1, it is necessary and sufficient that we should have
we are conducted, therefore, to the extended expression
which may be called an imaginary unit, because its modulus is = 1, and its square is negative unity. To distinguish one such imaginary unit from another, we may adopt the notation,
being still that point on the spheric surface which has
, , (or
, 
, 
)
for its rectangular coordinates; and then the formula of
multiplication (I) becomes, for any spherical triangle, in which
the rotation round , from to , is positive,
If 
be the positive pole of the arc
, or the pole to which the least rotation
from round is positive, then the product
of the two imaginary units in the first member of this formula
(which may be any two such units), is the following:
we have also, for the product of the same two factors, taken in the opposite order, the expression
which differs only in the sign of the imaginary part; and the product of these two products is unity, because, in general,
we have, therefore,
and the products
and 
may
be said to be reciprocals of each other.
In general, in virtue of the fundamental equations of definition, (A), (B), (C), although the distributive character of the multiplication of ordinary algebraic quantities (real or imaginary) extends to the operation of the same name in the theory of quaternions, so that
yet the commutative character is lost, and we cannot generally write for the new as for the old imaginaries,
since we have, for example, ji = - ij. However, in virtue of the same definitions, it will be found that another important property of the old multiplication is preserved, or extended to the new, namely, that which may be called the associative character of the operation, and which may have for its type the formula
thus we have, generally,
and so on for any number of factors; the notation
being employed to express that one
determined quaternion, which, in virtue of the theorem (Q), is
obtained, whether we first multiply as a
multiplicand by as a multiplier, and then multiply
the product 
as a multiplicand by
as a multiplier; or multiply first by , and
then by 
. With the help of this
principle, we might easily prove the equation (P), by observing
that its first member
.
In the same manner it is seen at once that
whatever n points upon the spheric surface may be denoted by
, , , 
,... 
:
and by combining this principle with that expressed by (M),
it is not difficult to prove that for any spherical polygon
, the following formula
holds good:
which includes the theorem (I') for the case of a spherical
triangle, and in which the arrangement of the n points may be supposed,
for simplicity, to be such that the rotations round from
to , round from
to , and so on, are all positive, and each less than
two right angles, though it is easy to interpret the expression
so as to include also the cases where any or all of these conditions
are violated. When the polygon becomes infinitely small, and therefore
plane, the imaginary units become all equal to each other, and may be
denoted by the common symbol i; and the formula (R) agrees then with
the known relation, that
Again, let , , be, respectively,
the representative points of any three quaternions
, , , and let
, 
,
be the representative points of the
three other quaternions, , ,
, derived by multiplication from the
former; then the algebraical principle expressed by the formula
(Q) may be geometrically enunciated by saying that the two points
and are the foci of a spherical
conic which touches the four sides of the spherical quadrilateral
; and analogous
theorems respecting spherical pentagons and other polygons may be
deduced, by constructing similarly the formulæ (Q'),&c.
In general, a quaternion , like an ordinary imaginary
quantity, may be put under the form,
provided that we assign to 
, or 
,
the extended meaning (L), which involves two arbitrary angles;
and the same general quaternion may be considered as a
root of a quadratic equation, with real coefficients, namely
which easily conducts to the following expression for a quotient, or formula for the division of quaternions,
if we define 
or
to mean that quaternion which gives the product
, when it is multiplied as a multiplicand by
as a multiplier. The same general formula
(S'') of division may easily be deduced from the
equation (O), by writing that equation as follows,
or it may be obtained from the four general equations of multiplication (D), by treating the four constituents of the multiplicand, namely w', x', y', z', as the four sought quantities, while w, x, y, z, and w'', x'', y'', z'', are given; or from a construction of spherical trigonometry, on principles already laid down.
The general expression (S) for a quaternion may be raised
to any power with a real exponent q, in the same manner
as an ordinary imaginary expression, by treating the square
root of -1 which it involves as an imaginary unit
having (in general) a fixed direction; raising
the modulus 
to the proposed real power; and multiplying
the amplitude , increased or diminished by any whole
number of circumferences, by the exponent q: thus
if q be real, and if n be any whole number. For example, a quaternion has in general two, and only two, different square roots, and they differ only in their signs, being both included in the formula,
in which it is useless to assign to n any other values than
0 and 1; although, in the particular case where the
original quaternion reduces itself to a real and negative quantity,
so that 
, this formula (T') becomes
the direction of remaining here entirely undetermined;
a result agreeing with the expression (L) or (L') for
. In like manner the quaternions, which are cube roots
of unity, are included in the expression
denoting here again an imaginary unit, with a direction
altogether arbitrary.
If we make, for abridgment
the series here indicated will be always convergent, whatever
quaternion may be; and we can always separate its
real and imaginary parts by the formula
which gives, reciprocally, for the inverse function 
,
the expression
u being any whole number, and 
being the natural,
or Napierian, logarithm of , or, in other words, that
real quantity, positive or negative, of which the function f
is equal to the given real and positive modulus . And
although the ordinary property of exponential functions, namely
does not in general hold good, in the present theory, unless
the two quaternions and be codirectional,
yet we may raise the function f to any real power by the formula
which it is natural to extend, by definition, to the case where the exponent q becomes itself a quaternion. The general equation,
when put under the form
will then give
and thus the general expression for a quaternion q, which is
one of the logarithms of a given quaternion 
to a given base 
, is found to involve two independent
whole numbers n and n', as in the theories of Graves and Ohm,
respecting the general logarithms of ordinary imaginary quantities
to ordinary imaginary bases.
For other developments and applications of the new theory, it is necessary to refer to the original paper from which this abstract is taken, and which will probably appear in the twenty-first volume of the Transactions of the Academy.
Links:
D.R. Wilkins