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(2) If OA' > 0A, cut off from OA', OB, O'C', OD lengths equal to 0A, and draw the inner quadrilateral as shown in the figure (XYZW). Then

AB > A'B' > XY,
CD> C'D' > ZW,
AD=AD > Wx,

BC= B'C'> YZ.
Consequently the same absurdity as in (1) follows a fortiori.
Therefore, since OA is neither equal to nor less than OA',

OA > OA'. The fact is also sufficiently clear if we draw MO, NO bisecting DA, DC perpendicularly and therefore meeting in O, the centre of the circumscribed circle, and then suppose the side DA with the perpendicular MO to turn inwards about D as centre. Then the intersection of MO and NO, as P, will gradually move towards N.

Simson gives his proof as “Lemma 11." immediately before XII. 17. He adds to the Porism some words explaining how we may construct a similar polyhedron in another sphere and how we may prove that the polyhedra are similar.

The Porism is of course of the essence of the matter because it is the porism which as much as the construction is wanted in the next proposition. It would therefore not have been amiss to include the Porism in the enunciation of XII. 17 so as to call attention to it.

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PROPOSITION 18.

Spheres are to one another in the triplicate ratio of their respective diameters.

Let the spheres ABC, DEF be conceived, and let BC, EF be their diameters; I say that the sphere ABC has to the sphere DEF the ratio triplicate of that which BC has to EF.

For, if the sphere ABC has not to the sphere DEF the ratio triplicate of that which BC has to EF, then the sphere ABC will have either to some less sphere than the sphere DEF, or to a greater, the ratio triplicate of that which BC has to EF.

First, let it have that ratio to a less sphere GHK, let DEF be conceived about the same centre with GHK, let there be inscribed in the greater sphere DEF a polyhedral solid which does not touch the lesser sphere GHK at its surface,

(XII. 17)

and let there also be inscribed in the sphere ABC a polyhedral solid similar to the polyhedral solid in the sphere DEF; therefore the polyhedral solid in ABC has to the polyhedral solid in DEF the ratio triplicate of that which BC has to EF.

[xii. 17, Por.)

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But the sphere ABC also has to the sphere GHK the ratio triplicate of that which BC has to EF; therefore, as the sphere ABC is to the sphere GHK, so is the polyhedral solid in the sphere ABC to the polyhedral solid in the sphere DEF; and, alternately, as the sphere ABC is to the polyhedron in it, so is the sphere GHK to the polyhedral solid in the sphere DEF.

(v. 16] But the sphere ABC is greater than the polyhedron in it; therefore the sphere GHK is also greater than the polyhedron in the sphere DEF.

But it is also less, for it is enclosed by it.

Therefore the sphere ABC has not to a less sphere than the sphere DEF the ratio triplicate of that which the diameter BC has to EF.

Similarly we can prove that neither has the sphere DEF to a less sphere than the sphere ABC the ratio triplicate of that which EF has to BC.

I say next that neither has the sphere ABC to any greater sphere than the sphere DEF the ratio triplicate of that which BC has to EF.

For, if possible, let it have that ratio to a greater, LMN; therefore, inversely, the sphere LMN has to the sphere ABC the ratio triplicate of that which the diameter EF has to the diameter BC.

But, inasmuch as LMN is greater than DEF, therefore, as the sphere LMN is to the sphere ABC, so is the sphere DEF to some less sphere than the sphere ABC, as was before proved.

(XII. 2, Lemma] Therefore the sphere DEF also has to some less sphere than the sphere ABC the ratio triplicate of that which EF has to BC: which was proved impossible.

Therefore the sphere ABC has not to any sphere greater than the sphere DEF the ratio triplicate of that which BC has to EF

But it was proved that neither has it that ratio to a less sphere.

Therefore the sphere ABC has to the sphere DEF the ratio triplicate of that which BC has to EF.

Q. E. D.

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It is the method of this proposition which Legendre adopted for his proof of xul. 2 (see note on that proposition).

The argument can be put very shortly. We will suppose S, S' to be the volumes of the spheres, and d, d' to be their diameters; and we will for brevity express the triplicate ratio of d to d' by du: d’3. If

d': d'3 S:S', then

ds: d'= S:T,
where T is the volume of some sphere either greater or less than S'.

I. Suppose, if possible, that T <S'.
Let T be supposed concentric with S'.

As in XII. 17, inscribe a polyhedron in S' such that its faces do not anywhere touch T; and inscribe in S a polyhedron similar to that in S'.

Then

S:T=du : d'3

= (polyhedron in S): (polyhedron in S'); or, alternately,

S: (polyhedron in S)= T: (polyhedron in S'). And

S> (polyhedron in S); therefore

T> (polyhedron in S').
But, by construction, T< (polyhedron in S'):
which is impossible.
Therefore

TS.
II. Suppose, if possible, that T >S'.
Now

d : d'3 = S:T

= X:S', where X is the volume of some sphere less than S,

(XII. 2, Lemma) or, inversely,

d'u: d = S': X, where X <S.

This is proved impossible exactly as in Part I.
Therefore

TS.
Hence T, not being greater or less than S', is equal to it, and

d: d'3 = S:S'.

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BOOK XIII.

HISTORICAL NOTE.

I have already given, in the note to iv. 10, the evidence upon which the construction of the five regular solids is attributed to the Pythagoreans. Some of them, the cube, the tetrahedron (which is nothing but a pyramid), and the octahedron (which is only a double pyramid with a square base), cannot but have been known to the Egyptians. And it appears that dodecahedra have been found, of bronze or other material, which may belong to periods earlier than Pythagoras' time by some centuries (for references see Cantor's Geschichte der Mathematik 13, pp. 175–6).

It is true that the author of the scholium No. 1 to Eucl. xiii. says that the Book is about “the five so-called Platonic figures, which however do not belong to Plato, three of the aforesaid five figures being due to the Pythagoreans, namely the cube, the pyramid and the dodecahedron, while the octahedron and the icosahedron are due to Theaetetus.” This statement (taken probably from Geminus) may perhaps rest on the fact that Theaetetus was the first to write at any length about the two last-mentioned solids. We are told indeed by Suidas (s. v. Ocaíontos) that Theaetetus “first wrote on the 'five solids' as they are called.” This no doubt means that Theaetetus was the first to write a complete and systematic treatise on all the regular solids; it does not exclude the possibility that Hippasus or others had already written on the dodecahedron. The fact that Theaetetus wrote upon the regular solids agrees very well with the evidence which we possess of his contributions to the theory of irrationals, the connexion between which and the investigation of the regular solids is seen in Euclid's Book xiii.

Theaetetus Aourished about 380 B.C., and his work on the regular solids was soon followed by another, that of Aristaeus, an elder contemporary of Euclid, who also wrote an important book on Solid Loci, i.e. on conics treated as loci. This Aristaeus (known as “the elder”) wrote in the period about 320 B.C. We hear of his Comparison of the five regular solids from Hypsicles (2nd cent. B.c.), the writer of the short book commonly included in the editions of the Elements as Book xiv. Hypsicles gives in this Book some six propositions supplementing Eucl. xii.; and he introduces the second of the propositions (Heiberg's Euclid, Vol. v. p. 6) as follows:

The same circle circumscribes both the pentagon of the dodecahedron and the triangle of the icosahedron when both are inscribed in the same sphere. This is proved by Aristaeus in the book entitled Comparison of the five figures."

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