The color dispersion of an optical medium is estimated by its dispersive power, which is de fined by the following ratio: where the symbols nc,aD, nr, denote the magni tudes of the indices of refraction for light cor responding to the Fraunhofer lines •C (red), D (yellow), F (blue), respectively. The reciprocal of the dispersive power of the more important varieties of glass used in the manu facture of optical instruments varies between the limits 70 and 30.
Optical Images.— Generally, when an ob ject is viewed, directly or indirectly, not all or even most of the rays which are emitted are utilized for the purpose of vision. In fact, it is usual to interpose in front of the luminous object an opaque screen with a suitable opening in it for the express purpose of intercepting the needless or undesirable rays and permitting the others, called the rays, to pass unhampered through t e opening. This is admirably exempli fied by the iris opening in the human eye which enables the observer to utilize only those rays which are best adapted to the organ of vision, so that from each point of the object in the field of view there comes into the eye a bundle of effective rays whose aperture is de termined by the dimensions of the pupil. The organ of vision is subject to optical illusions in regard to the external world, due to various causes, among others to the fact that the eye judges the object to be at the place whence the rays emanate just before entering the eye, which may not correspond to the place where the object actually is. Thus, for example, if the bundle of rays proceeding originally from a luminous point of the object has been modi fied by reflection or refraction before entering the eye, the object itself will not be visible, and at best the eye can perceive only a so called image of the object, which in general will be blurred and distorted and situated dif ferently from the object as to both distance and direction. In the singular and very special case when the effective rays after emerging from the optical system of isotropic media, which are here supposed to be interposed be tween the object and the eye, all meet again in one point, there will be formed at this point an ideal optical image of the luminous object point. And even when the image is not abso lutely ideal, it may be so nearly perfect that the limited resolving power of the human eye i is not able to discern its faults, so that then the image is practically ideal.
Nearly all optical instruments are contriv ances for utilizing the phenomena of light in order to reproduce as distinctly and faithfully as possible images of luminous objects. Such
instruments (e.g., microscope, telescope, photo graphic lens, etc.) are usually composed of a single lens or combination of lenses. The word lens is used in optics to denote a portion of a transparent substance, usually isotropic and most often made of glass, comprised between two smooth polished faces, one of which may be plane. (If both faces are plane, it is not a lens but a prism — or a slab, in case the two plane faces are parallel). The faces of the lens arc generally, but not necessarily, spherical in form. The optical axis of a lens is a straight line which is normal to both faces, and a ray whose path lies along the axis (the axial ray) will pass through the lens without being deflected from this line. A lens is said to be convex or concave according as it is thicker or thinner, respectively, in the middle along the axis than out toward the edges. A convex lens may be double convex (Fig. 2, a), plano-convex (Fig. 2, b) or a convex meniscus (Fig. 2, c); similarly, a concave lens may be double concave (Fig. 3, a), plano-concave (Fig. 3, b) or a concave meniscus (Fig. 3, c). A convex glass lens of moderate thickness, sur rounded by air, will be found to converge a beam of parallel rays of sunlight to a focus on the far side of the lens; and hence it is said to be a convergent or positive lens. On the other hand, a concave lens under the same circumstances will render a beam of sunlight divergent, and hence it is called a divergent or negative lens.
The lenses in an optical instrument are al most invariably disposed so that the centres of the spherical refracting surfaces lie all on one straight line, which is an axis of symmetry (optical axis). The instrument is said to di vide the surrounding region into two parts known as object-space and image-space; which, however, are not to be regarded as separate and distinct parts of space, but rather as names to be applied, to different aspects of the same space; whereby any point or straight line may be considered as belonging equally to either region depending merely on the point of view. The effective rays before they enter the instrument are called object-rays, since they have their origins in the luminous object; whereas the emergent rays which go to form the more or less imperfect image are called image-rays.