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In point of fact, the field of view is not so large as represented here, for the point p receives but one single ray that can fall on the eye glass, and therefore will not be visible.
If the opposite extremities of the glasses be joined by lines mn, Nm, they will bound a part of the image tqs, which transmits all its light.
It will be seen from the figure, that the eye must be placed at a little distance from the eye glass to receive the rays proceeding from the extreme verge of the field of view.
168. The image is inverted as in the compound microscope. It may be set upright by an additional pair of lenses C, D, (Fig. 180), which are placed so as to have a common focus, and, usually, have no effect on the magnifying power.
This construction is used in what are called Day Telescopes, which are chiefly employed for viewing distant terrestrial objects; for observations on the heavenly bodies, the additional glasses are dispensed with, in order to save the light that is lost by the two additional refractions.
The lenses in the figures are drawn of their full diameter, but in practice it is usual to limit their apertures in order to diminish the aberrations, (see Fig. 181). In this case we must consider the lenses as extending no farther than these apertures.
169. In telescopes to be used for astronomical observations it is usual to put a net-work of fine wire or sometimes of spider's web at the focus of the object glass, in order to determine the precise position of a star as it passes by them. This apparatus is called a Micrometer, and its simplest form is represented in Fig. 182, having five parallel wires dividing the diameter of a circular diaphragm into equal parts, and a sixth bisecting them all perpendicularly.
Another kind of wire micrometer consists of two parallel wires, the one fixed, and the other moveable by means of a fine screw, with a third perpendicular to them. This is represented in Fig. 183; it is used for measuring the apparent diameters of the heavenly bodies.
