The explanation of the difference of focus upon the two sides as due to unequal spacing was verified by Cornu upon gratings purposely constructed with an increasing interval.
In this the tongue of the relay is kept over to the spacing side by means of a current flowing in one direction, but on the depression of the signalling key the current is reversed, moving the relay tongue over to the marking side.
The clear, noble spacing of the surface work is well shown by a group of offerings and inscribed titles (Plate III.
Beyond the XIIth Dynasty estimates must again be vague rhe spacing of the years on the Palermo stone has given rise to some calculations for the early dynasties.
The spacing, or distance from centre to centre of the floor beams, will depend upon the type of fire-proof flooring employed; it also depends to a considerable extent upon the amount and character of the floor load and the length of span.
The irregularity of spacing has thus the effect of a convex lens, which accelerates the marginal relatively to the central rays.
A linear error in the spacing, and a general curvature of the lines, are eliminated in the ordinary use of a grating.