On Quaternions

Sir William Rowan Hamilton

[Proceedings of the Royal Irish Academy, Nov. 11, 1844,
vol. 3 (1847), 1-16]

In the theory which Sir William Hamilton submitted to the Academy in November, 1843, the name quaternion was employed to denote a certain quadrinomial expression, of which one term was called (by analogy to the language of ordinary algebra) the real part, while the three other terms made up together a trinomial, which (by the same analogy) was called the imaginary part of the quaternion: the square of the former part (or term) being always a positive, but the square of the latter part (or trinomial) being always a negative quantity. More particularly, this imaginary trinomial was of the form ix + jy + kz, in which x, y, z were three real and independent coefficients, or constituents, and were, in several applications of the theory, constructed or represented by three rectangular coordinates; while i, j, k were certain imaginary units, or symbols, subject to the following laws of combination as regards their squares and products,




but were entirely free from any linear relation among themselves; in such a manner, that to establish an equation between two such imaginary trinomials was to equate each of the three constituents, xyz, of the one to the corresponding constituent of the other; and to equate two quaternions was (in general) to establish FOUR separate and distinct equations between real quantities. Operations on such quaternions were performed, as far as possible, according to the analogies of ordinary algebra; the distributive property of multiplication, and another, which may be called the associative property of that operation, being, for example, retained: with one important departure, however, from the received rules of calculation, arising from the abandonment of the commutative property of multiplication, as not in general holding good for the mixture of the new imaginaries; since the product ji (for example) has, by its definition, a different sign from ij. And several constructions and conclusions, especially as respected the geometry of the sphere, were drawn from these principles, of which some have since been printed among the Proceedings of the Academy for the date already referred to.


Thus he has found it useful, in many applications, to dismiss the separate consideration of the three real constituents, x, y, z, of the imaginary trinomial ix + jy + kz, and to denote that trinomial by some single letter (taken often from the Greek alphabet). And on account of the facility with which this so called imaginary expression, or square root of a negative quantity, is constructed by a right line having direction in space, and having x, y, z for its three rectangular components, or projections on three rectangular axes, he has been induced to call the trinomial expression itself, as well as the line which it represents, a VECTOR. A quaternion may thus be said to consist generally of a real part and a vector. The fixing a special attention on this part, or element, of a quaternion, by giving it a special name, and denoting it in many calculations by a single and special sign, appears to the author to have been an improvement in his method of dealing with the subject: although the general notion of treating the constituents of the imaginary part as coordinates had occurred to him in his first researches.


By the fundamental equations of definition for the squares and products of the symbols i, j, k, it is easy to see that any (so-called) real and positive quantity is to any vector whatever, as that vector is to a certain real and negative quantity; this being indeed only another mode of saying that, in this theory, every vector has a negative square. Again, the product of any two rectangular vectors is a third vector at right angles to both the factors (but having one or other of two opposite directions, according to the order in which those factors are taken); a relation which may be expressed by saying, that the fourth proportional to the real unit and to any two rectangular vectors is a third vector rectangular to both; or, conversely, that the fourth proportional to any three rectangular vectors is a quantity distinct from every vector, and of the kind called real in this theory, as contrasted with the kind called imaginary.

Now, in fact, what originally led the author of the present communication to conceive (in 1843) his theory of quaternions (though he had, at a date earlier by several years, speculated on triplets and sets of numbers, as an extension of the theory of couples, or of the ordinary imaginaries of algebra, and also as an additional illustration of his view respecting the Science of Pure Time), was a desire to form to himself a distinct conception, and to find a manageable algebraical expression, of a fourth proportional to three rectangular lines, when the DIRECTIONS of those lines were taken into account; as Mr. Warren and Mr. Peacock had shewn how to conceive and express the fourth proportional to any three lines having direction, but situated in one common plane. And it has since appeared to Sir William Hamilton that the subject of quaternions may be illustrated by considering more closely, though briefly, this question of the determination of a fourth proportional to three rectangular directions in space, rather in a geometrical than in an algebraical point of view.

Adopting the known results above referred to, for proportions between lines having direction in a single plane (though varying a little the known manner of speaking on the subject), it may be said that, in the horizontal plane, ``West is to South as South is to East,'' and generally as any direction is to one less advanced than itself in azimuth by ninety degrees. Let it be now assumed, as an extension of this view, that in some analogous sense there exists a fourth proportional to the three rectangular directions, West, South, and Up; and let this be called, provisionally, Forward, by contrast to the opposite direction, Backward, which must be assumed to be (in the same general sense) a fourth proportional to the directions of West, South and Down. We shall then have, inversely, Forward to Up as South to West, and therefore, as West to North: if we admit, as it seems natural and almost necessary to do, that (for directions, as for lengths) the inverses of equal ratios are equal; and that ratios equal to the same ratio are equal to each other. But again, Up is to South as South to Down, and also as North to Up: and we can scarcely avoid admitting, or defining, that (in the present comparison of directions) ratios similarly compounded of equal ratios are to be considered as being themselves equal ratios. Compounding, therefore, on the one hand, the ratios of Forward to Up, and of Up to South; and on the other hand the respectively equal (or similar) ratios of West to North, and of North to Up, we are conducted to admit that Forward is to South as West to Up. By a reasoning exactly similar, we find that Forward is to West as Up to South; and generally that if X, Y, Z denote any three rectangular directions such that A:X::Y:Z, A here denoting what we have expressed by the word Forward, then also A:Y::Z:X (and of course, for the same reason A:Z::X:Y); so that the three directions tex2html_wrap_inline342 may be all changed together by advancing them in a ternary cycle, according to the formula just written, without disturbing the proportionality assumed. But also, by the principle respecting proportions of directions in one plane, we may cause any two of the three rectangular directions XYZ to revolve together round the third, as round an axis, without altering their ratio to each other. And by combining these two principles, it is not difficult to see that because Forward has been supposed to be to Up as South to West, therefore the same (as yet unknown) direction ``Forward'' must be supposed to be to any direction X whatever, as any direction Y, perpendicular to X, is to that third direction Z which is perpendicular to both X and Y, and which is obtained from Y by a right-handed (and not by a left-handed) rotation, through a right angle, round X; in the same manner as (and because) the direction West was so chosen as to be to the right of South, with reference to Up as an axis of rotation. Conversely we must suppose that if any three rectangular directions, tex2html_wrap_inline342 , be arranged, as to order of rotation, in the manner just now stated, then Z:Y::X:A; or in other words, we must admit, if we reason in this way at all, that the direction called already Forward, will be the fourth proportional to tex2html_wrap_inline366 . And if we vary the order, so as to have Z to the left, and not to the right of Y, with reference to X, then will the fourth proportional to tex2html_wrap_inline366 become the direction which we have lately called Backward, as being the opposite to that named Forward.


The geometrical study of the relations between directions in space, combined with a few very simple and guiding principles respecting the composition of relations generally, might therefore have led to the conception or assumption of a certain pair of contrasted directions, namely, those which we have called Forward and Backward, and denoted by the letters A and B. And these are such that if we conceive a quantitative element to be combined with each, and give the name of POSITIVE UNITY to the unit of magnitude measured in the direction of Forward, but that of Negative Unity to the same magnitude measured backward; and if we extend to this positive unity and to lines having direction in space the received definitions of multiplication, that ``Positive Unity is to Multiplier as Multiplicand is to Product,'' and that ``the product of two equal factors is the square of either;'' we may then consistently and naturally be led to assert the same results as those already enunciated from the theory of quaternions respecting the product of two vectors, in the two principal cases, first, where those two vectors are rectangular, and second, where they are coincident with each other. And thus may we justify, or at least interpret and explain, the fundamental definitions (A) (B) (C) of this theory, by regarding the symbols tex2html_wrap_inline386 as denoting three vector-units having three rectangular directions in space.

But, farther, we derive from this view of the whole subject an illustration (if not a confirmation) of the remarkable conclusion that the so-called real and positive unit +1 is not (in this theory) to be confounded with any vector unit whatever, but is to be regarded as of a kind essentially distinct from every vector. For this positive unit +1 is in the direction above called Forward, and denoted by A. Now if this could coincide with a direction X in tridimensional space, then, whatever this latter direction might be supposed to be, we could always, by the general formula A:X::Y:Z (where X is arbitrary), deduce the inadmissible proportion X:X::Y:Z, in which the two directions in one ratio are identical, but those in the other are rectangular to each other. If then we resolve to retain the assumption of the existence of a fourth proportional A to three rectangular directions in space, as subject to be reasoned on at all in the way already described, and as determined in direction by its contrast to its own opposite B (corresponding to an opposite order of rotation in the system tex2html_wrap_inline342 ), we must think of these two opposite directions A and B as merely laid down upon a scale, but must abstain from attributing to this SCALE any one direction rather than another in tridimensional space, as having such or such a zenith distance, or such or such an azimuth, rather than such or such another. And the progression on this scale from negative to positive infinity, obtained by combining a quantitative element with the contrast between two opposite directions, corresponds less to the conception of space itself (though we have seen that considerations of space might have suggested it) than to the conception of time; the variety which it admits is not tri- by uni- dimensional; and it would, in the language of some philosophical systems, be said to appertain rather to the notion of intensive than of extensive magnitude. Though answering precisely to the progression of the quantities called real in algebra, it has, when viewed from the geometrical side, somewhat the same sort of imaginariness, and yet (it is believed) of utility, as compared with lines in space, which the square root of an ordinary negative has, when compared with positive and negative quantities. This analogy becomes still more complete when we observe that (in this theory) the fourth proportional to any direction X in space, and either of the two directions A or B upon the scale, is the direction opposite to X; so that, if a vector-unit in any determined direction X had been taken for positive unity, then each of the two scalar units in the directions A and B (in common, it is true, with every vector-unit perpendicular to X) might have been called, by the general nomenclature of multiplication, a square root of negative one.


As preliminary illustrations of the operations employed, it may be remarked that for any system of lines having direction in space, it is required by many analogies (and is, for lines in one plane included among the definitions or results of the theories of Mr. Warren and Mr. Peacock), that the sum should be regarded as being equal to that one line which constructs or represents the total effect of all the different rectilinear motions which are expressed by the different summands. Vectors are therefore to be added to each other by a certain geometrical composition, exactly analogous to the composition of motions, or of forces, and following the same known rules. Scalars, on the other hand (that is to say, the so-called real parts of any proposed quaternions), admitting only of a progression in quantity, and of a change of sign, without any other changes of direction, are to be added among themselves by the known rules of algebra, for the addition of positive and negative numbers. The addition of a scalar and a vector to each other can be no otherwise performed, or rather indicated, than by writing their symbols with the + sign interposed; each being, as we have seen, in some sense, imaginary with respect to the other. These operations of addition are all of the commutative, and also of the associative kind; that is to say, the order of all the summands may be changed, and any group of them may be collected or associated into one partial sum.


For the same reason, to multiply any vector by any scalar a, is in general to change its length in a known ratio, and to preserve or reverse its direction, according as a is > or < 0; the product is therefore a new vector, which may be denoted by tex2html_wrap_inline438 . The same new vector is obtained, under the form tex2html_wrap_inline440 , when we multiply the scalar a by the vector tex2html_wrap_inline444 . If tex2html_wrap_inline446 and tex2html_wrap_inline448 be two quaternion factors, of which a and b are the scalar parts, and tex2html_wrap_inline444 , tex2html_wrap_inline456 the vectors, then with a view to preserving the distributive character of multiplication, it is natural to define that the product may be distributed into the four following parts:


And if the multiplicand vector tex2html_wrap_inline456 be decomposed into two parts, or summands, one tex2html_wrap_inline460 and in the direction of the multiplier tex2html_wrap_inline444 , or in the direction exactly opposite thereto, and the other tex2html_wrap_inline464 , and in a direction perpendicular to the former (so that tex2html_wrap_inline466 and tex2html_wrap_inline468 are the projections of tex2html_wrap_inline456 on tex2html_wrap_inline444 itself, and on the plane perpendicular to tex2html_wrap_inline444 ), then it may be farther defined that the multiplication of any one vector  tex2html_wrap_inline456 by any other vector  tex2html_wrap_inline444 may be accomplished by the formula


in which, by what has been shewn, the partial product tex2html_wrap_inline480 is to be considered as equal to a scalar, namely, the product of the lengths of tex2html_wrap_inline444 and tex2html_wrap_inline466 , taken with the sign - or +, according as the direction of tex2html_wrap_inline466 coincides with, or is opposite to that of tex2html_wrap_inline444 ; while the other partial product tex2html_wrap_inline494 is a vector, of which the length is the product of the lengths of tex2html_wrap_inline444 and tex2html_wrap_inline468 , while its direction is perpendicular to both of their's, being obtained from that of tex2html_wrap_inline468 , by making it revolve right-handedly through a right angle round tex2html_wrap_inline444 as an axis. These definitions, which are compatible with the formulæ (A), (B), (C), and may serve to replace them, will be found sufficient to prove generally, and perhaps with somewhat greater geometrical clearness than those formulæ, the distributive and associative properties of quaternion multiplication, which have already been stated to exist. They give easily the following corollaries, which are of very frequent use in this calculus:



A and B denoting here the lengths of the lines tex2html_wrap_inline444 and tex2html_wrap_inline456 , and (A,B) the angle between them; while tex2html_wrap_inline514 is a vector-unit perpendicular to their plane, and such that a right-handed rotation, equal to the angle (A,B), performed round tex2html_wrap_inline514 , would bring the direction of tex2html_wrap_inline444 to coincide with that of tex2html_wrap_inline456 . For example, when tex2html_wrap_inline524 , then B = A, (A,B) = 0, and


so that the length A of any vector  tex2html_wrap_inline444 , in this theory, may be expressed under the form


More generally we have the equation


when the lines tex2html_wrap_inline444 and tex2html_wrap_inline456 are coincident or opposite in direction; while, on the contrary, the condition for their being at right angles to each other is expressed by the formula




will give, by being squared,




a known result, it is true, but one which can scarcely be derived in any other way by so very short a process of calculation. For it is not quite so easy, on the algebraical side of the question, to see that


however easy this may be, as it is to see that


although the geometrical interpretation of the first of these two formulæ is of course more obvious than that of the latter, to those who are familiar with the method of coordinates, and not with the method of quaternions.

Again, let us consider the more difficult problem of the composition of any number os successive rotations of a body, or, at first, of any one line thereof, round several successive axes, through any angles, small or large. Let the axis of the first of these rotations have the direction of the vector-unit tex2html_wrap_inline444 , ( tex2html_wrap_inline548 ), and let the amount of the positive rotation round this axis be denoted by a, which letter here represents still a scalar or real number. Let tex2html_wrap_inline456 be the revolving line, considered in its original position; tex2html_wrap_inline554 the same line, after it has revolved through the angle a round the axis  tex2html_wrap_inline444 . The part, or component, of tex2html_wrap_inline456 , which is in the direction of this axis, is that which was denoted lately by tex2html_wrap_inline466 ; and the formula (a), when multiplied by tex2html_wrap_inline564 , gives, as an expression for this part,


because it has been supposed that tex2html_wrap_inline548 . This part of tex2html_wrap_inline456 remains unaltered by the rotation. The other part, or component of tex2html_wrap_inline456 , is, in like manner, by (b),


and this part is to be multiplied by tex2html_wrap_inline572 , in order to find the part of tex2html_wrap_inline554 , which is perpendicular to tex2html_wrap_inline444 , but in the plane of tex2html_wrap_inline444 and tex2html_wrap_inline456 . Again, multiplying by tex2html_wrap_inline444 , we cause tex2html_wrap_inline468 to turn through a right angle in the positive direction round  tex2html_wrap_inline444 , and obtain, for the result of this rotation,


an expression which is the half of that marked (b), and which is to be multiplied by tex2html_wrap_inline588 , in order to arrive at the remaining part of the sought line tex2html_wrap_inline554 , namely, the part which is perpendicular to the plane of tex2html_wrap_inline444 and tex2html_wrap_inline456 . Collecting, therefore, the three parts, or terms, which have been thus separately obtained, we find,


that is,


[Note added during printing.]

the operations here indicated being thus sure to make no change in the part tex2html_wrap_inline466 , which is in the direction of the axis of rotation, but to cause the other part tex2html_wrap_inline468 to revolve round that axis  tex2html_wrap_inline444 through an angle = a. Again, let the same line tex2html_wrap_inline554 revolve round a new axis of rotation denoted by a new vector unit tex2html_wrap_inline636 , through a new angle a', into a new position tex2html_wrap_inline640 ; we shall have, in like manner,


and so on, for any number n of rotations. Let the last position of tex2html_wrap_inline456 be denoted by tex2html_wrap_inline646 ; and since it can easily be proved, by the theory of multiplication of quaternions, that the continued products which present themselves admit of being thus transformed:


in which tex2html_wrap_inline648 is a new vector unit, and tex2html_wrap_inline650 a new real angle, we find that the result of all the n rotations is of the form


It conducts, therefore to the same final position which would have been attained from the initial position  tex2html_wrap_inline456 , by a single rotation tex2html_wrap_inline656 , round the single axis  tex2html_wrap_inline648 ; the amount and axis of this resultant rotation being determined by either of the two equations of transformation (k), and being independent of the direction of the line  tex2html_wrap_inline456 which was operated on, so that they are the same for all lines of the body.




D.R. Wilkins
School of Mathematics
Trinity College, Dublin