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. [Illustration: FIG. 30(a) FIG. 30(b) FIG. 30(c)] 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 30_b_ and 30_c_. 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 30_b_ and 30_c_ 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 30_b_, 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°. 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.