For reading signals at short ranges, say, up to 5 miles, these glasses are better than the telescope. Flag signals have frequently been read with glasses of this description at a distance of 10 miles.
Light.—The illumination of an object when observed with the unaided eye is impressed upon the retina with a brightness in strict proportion to that of the object itself. If an object be viewed under equal illuminating conditions alternately with the naked eye and with a glass, the brightness of the image seen with the naked eye may be represented by 1, while that of the image in the glass will generally differ, being greater or less.
The light of the telescope or field glass is expressed by the number which shows how many times brighter the object appears through the instrument than to the naked eye. Light is a function of the dimensions of the object glass and of the power of the instrument, and is sometimes determined by dividing the square of the objective aperture (expressed in millimeters) by the square of the power.
The light of a telescope or field glass can also be determined by means of the absorption apparatus shown in [figure 30] (a) (b) (c).
This absorption apparatus operates on the principle of viewing an object through a perfectly black liquid, which absorbs all colors equally, and of increasing the thickness of the liquid layer until the object becomes invisible. The thickness of the layer of liquid will then be a measure of the relative brightness or intensity of the illumination.
The apparatus consists of two wedge-shaped vessels, made of brass, with glass windows in the sides. One of these vessels is shown in perspective in [figure 30]a. The sides A and the one opposite are of glass. B is tubulure for filling the apparatus, and is stopped with a cap. The operation of the apparatus is shown diagrammatically in [figures 30b] and [30c]. The edges of the two wedges which come together are divided into scales of equal parts of convenient magnitude. Each scale begins with zero; not at the extreme point of the wedge outside, but at a point, which, allowing for the thickness of the glass sides, is opposite the point of the wedge of liquid inside. It will be observed in figures 30b and 30c that the sum of any two adjacent numbers, on the respective scales, over the whole overlapping portion of the wedges, is the same. Thus in [figure 30]b it is 11, and in [figure 30]c it is 7. These figures measure the relative thickness of the liquid layers in the two respective settings of the apparatus. Suppose the image is just obliterated, when looking with the unaided eye, at the setting shown in figure 30b, and when using the glass at the setting shown in [figure 30]c. This would mean that the illuminating power of the glass is seven-elevenths. In using the apparatus, a focusing cloth, used by all photographers, is useful in excluding stray light.
Field.—Maintaining the head and eyes as motionless as possible, the field of vision of the unaided eye or the range within which objects can be perceived by the unaided eye varies according to direction.
De Schweinitz gives the following limits: Outward, 90°; outward and upward, 70°; upward, 50°; upward and inward, 55°; inward, 60°; inward and downward, 55°; downward, 72°; downward and outward, 85°.