By keeping watch on Toulon that squadron covered not only the Straits, but also the focal points within the sea.
This would also be about the focal length of the finished lens.
In an ellipse the sum of the focal distances is constant; and in an hyperbola the difference of the focal distances is constant.
Toward a focal point: from the east, two people; from the north, two people.
For every voluntary act of sight there are two adjustments of the eyes, namely, focal and axial.
B is a double convex lens, which has a focal length of twenty inches.
It is naturally the focal point of the trade between Europe and Asia.
It should not be forgotten that it is, in a sense, the focal centre of the room.
Thus if the aperture is one eighth the focal length, it is marked f/8, if a sixteenth f/16, and so on.
This is the only apparatus of this focal distance on the French coast.
focal 1690s, from focus + -al (1).