It may be safely said that the field or "visual angle" of the unaided eye for distinct vision is at least 45° in all directions.
The "visual angle" or "field" of a field glass is always smaller, no field glass having yet been designed which could equal the field of the unaided eye.
The field of a telescope or field glass can best be determined by the use of a transit or other instrument used in measuring horizontal angles. The glass is placed upon the telescope of the transit in such a way that the axes of collimation of the transit and the telescope or field glass are parallel. The extreme limits of the field of view are marked and the horizontal angle between the markers noted on the limb of the transit.
Definition.—One of the chief qualities of the eye is its power of defining outlines and details distinctly. Relative characteristics in this respect may be determined in various ways. Thus the distance at which printed matter can be read, or the details of a distant object distinguished, will give a fair measure of the defining power of the eye; but a better method is to express the definition of sight by angular measurement—that is, by the determination of the smallest visual angle giving clear results. Experience teaches that this angle of the normal eye (with good light and favorable color conditions) is about 40′′, and it is therefore possible to determine the smallest object which can just be seen, well defined, at an arbitrary distance. For instance, at a distance of 15 feet an object can be seen which is one-twentieth of an inch high or broad; at 30 feet distance, consequently, the object must be twice the size (one-tenth of an inch) to be seen, and so on relatively, within limits, as distance increases. But as the distance becomes greater sharpness of vision is impaired materially by the interposing atmosphere, while it is also affected by color contrasts and conditions of illumination. It therefore follows that at considerable distances objects which subtend a visual angle of 40′′ are no longer clearly defined but become so only as the angle approaches 60′′, 120′′, 180′′, or more.
The most important and essential quality of a telescope or field glass is definition, i. e., the sharpness, clearness, and the purity of the images seen through it. To obtain good definition it is necessary that spherical and chromatic aberration be overcome, that the polish of the lenses be as perfect as possible, that the cement possess no inequalities, and that the lenses be well focused, that there be no dampness in the interior of the tubes, and, generally, that the instrument be without optical defect.
Faults in this direction are discovered at once by examination of definition, whereas in determining the other constants they are less noticeable. In comparing the definition of any two instruments it is ordinarily necessary only to scan distant objects and observe to what extent details may be distinguished.
The following test may also be used: Focus on printed matter at a distance just beyond that at which perfect clearness is given and gradually approach until the letters are distinctly defined. The instrument with which the print can be read at the greatest distance has the best definition.
To express definition as an absolute measure, use instead of printed matter, a white sheet of paper upon which a series of heavy lines are drawn at intervals equivalent to their thickness. Focus upon this and gradually approach from a point where the impression of a uniform gray field ceases and the black lines and white intervals begin to appear distinct and defined.
Let the distance thus found be 20 yards and the thickness of the lines and intervals between them one-tenth inch. The circumference of a circle with a radius of 20 yards or 7,200 tenths inches is 14,400 by 3.1416 or 45,240 tenth inches; but a circumference equals 360° or (360 by 60 by 60) 1,296,000′′.