5) is QQ'- **QS** = BD sec 4:,(I - cos SQQ') = BD sec cI)(1 +cos 20) = 2BD cos 4); from which it follows that an error of given magnitude in the figure of a surface is less important in oblique than in perpendicular reflection.

Apra p rts, 'Ap-ro pEns, and in an inscription of Tralles (Dittenberger, Sylloge, 573) Apra o o **qs**; Herodotus (vi.

The conditions (~) then lead to IA(AC) 2, ,2 (AC)(BC) 1 ~ tO **qs** B(BC)~ AB r0, C(BC) r~

It is implied in the above description of the system that the Cartesian co-ordinates x, y, z of any particle of the system are known functions of the **qs**, varying in form (of course) from particle to particle.

The coefficients arr, a,~, are called the coefficients of inertia; they are not in general constants, being functions of the **qs** and so variable with the configuration.

This solution, taken by itself, represents a motion in which each particle of the system (since its displacements parallel to Cartesian co-ordinate axes are linear functions of the **qs**) executes a simple vibration of period 21r/u.

Where the denominator stands for the same homogeneou~ quadratic function of the **qs** that T is for the is.

If r be the number of quotients in the recurring cycle, we can by writing down the relations connectin g the successive p's and q's obtain a linear relation connecting p nr +m, t'(n-1)r +m, +m in which the coefficients are all constants.