The classification of animals and plants serves two important purposes: one is that the structural peculiarities and affinities of the groups may be contrasted and a knowledge of their absolute and their differential characters acquired, and for this a natural system is eminently serviceable; the second is that of enabling any animal or plant to be simply distinguished from any other, for which an artificial or analytical system is extremely useful.

CHAPTER XIV.
OPTICAL PRINCIPLES.

WE shall now devote a few pages to the consideration of the nature of light, and the optical principles involved in the construction and use of the microscope. Two theories of light have been propounded. According to one, light consists of minute particles emanating from self-luminous bodies, as the sun, a candle, or a red-hot piece of iron; this is called the corpuscular theory. According to the other, light consists of waves or undulations like those of water or the ears of corn set in motion by the wind, of the molecules of an extremely subtle and rarified elastic matter, called ether, existing everywhere, and set in motion by the causes which produce light; this is called the undulatory theory. The consideration of the merits of these two theories would be foreign to our purpose: suffice it to say that the evidence in favour of the undulatory theory preponderates, so that the corpuscular theory is now laid aside.

It will often be requisite to make use of the term ray of light, by which must be understood the smallest bundle of luminous undulations which can be separated from a mass of light—as by passing light through a small hole in an opake body, or by any equivalent method.

The most casual observer must have noticed that the rays of light move in straight lines; as when the sun’s rays are seen entering a dark room through a small window or other aperture, their direction being then distinctly visible; the manner in which ordinary shadows are formed also illustrates the same fact.

Refraction.—But when the rays in their passage impinge or are incident upon and enter a transparent medium or material, of a different density from that which they were at first traversing, their course becomes altered, and the line of their direction broken, whence they are said to be refracted. If the medium upon which the rays impinge be denser than that through which they were at first passing, they will be refracted towards a line perpendicular to the surface, or they will be refracted towards the perpendicular, as it is expressed.

Thus, as shown in [Pl. XII.] fig. 1, the incident ray i, entering the plate of glass, will be refracted at its surface in the direction a r, towards the line p, which is perpendicular to the surface.

The extent to which the rays undergo refraction depends upon the degree of density of the medium, and varies in the case of each individual substance; but it follows a definite law. If, as in [Pl. XII.] fig. 2, a circle be drawn around the point b, at which the ray a is incident, b r representing the refracted ray, the lines s i and t r, drawn at right angles to the perpendicular p, will form respectively the sines, as they are called, of the angles s b i and t b r; s i being the sine of the angle of incidence s b i, or the angle formed by the incident ray with the perpendicular, and t r the sine of the angle of refraction t b r, or of that formed by the refracted ray with the perpendicular. These sines, for brevity, are called the sines of incidence and of refraction; and they bear a constant ratio or proportion to each other. Taking the sine of refraction as the unit, or as = 1, the value of the sine of incidence represents the refractive index or the refractive power of the medium for a ray entering the medium from a vacuum; or, the refractive power of air being extremely small, the value of the sine of incidence may be considered as representing the refractive power from air into the medium.

PLATE XII. [[PAGE 168.]]
Optical principles.
Fig.
1.Refraction through a glass plate.
2.Law of refraction.
3.Reflexion from a plane surface.
4.Reflexion from a concave mirror.
5.Refraction at a curved surface.
6.A doubly convex lens.
7.A plano-convex lens.
8.A doubly concave lens.
9.A plano-concave lens.
10.A concavo-convex lens, or meniscus.
11.Refraction through a convex lens.
12.Relation of a convex lens to prisms.
13.Relation of a concave lens to prisms.
14.Refraction of parallel rays through a convex lens.
15.Refraction of converging rays through a convex lens.
16.Refraction of diverging rays through a convex lens.
17.Refraction of parallel rays through a concave lens.
18.Spherical aberration.
19.Dispersion and formation of a spectrum.
20.Chromatic aberration.
21.Formation of images in the eye.
22.Angle of vision.
23.Objects too near the eye.
24.Action of convex lens in vision.
25.Aplanatism.
26.Aberration produced by cover.
27.Course of rays through the microscope.
28.Achromatism.
29.Waves of light conspiring (a, b), and interfering (b, c).
30.Polarization: t, tourmaline; d, crystal; s, crystal of calcareous spar.