A pair of contact springs are slowly shifted over so as to close the circuit at successive assigned instants during a complete phase.
In the instants when a sharp image of the photosphere is seen or photographed, it shows a granulated appearance like white flakes strewed fairly evenly upon a dark ground.
When a machine undergoes alternate acceleration and retardation, so that at certain instants of time, occurring at the end of intervals called periods or cycles, it returns to its original speed, then in each of those periods or cycles the alternate excesses of energy and of work neutralize each other; and at the end of each cycle the principle of the equality of energy and work stated in 87, with all, its consequences, is verified exactly as in the case of machines of uniform speed.
At intermediate instants, however, other principles have also to be taken into account, which are deduced from the second law of motion, as applied to direct deviation, or acceleration and retardation.
The instants of contact between the limbs of the sun and planet defied precise determination.