ON the construction of the modern microscope, optical and mechanical principles of some importance are involved. Theses principles we shall briefly explain. Together with the more recent improvements effected in the instrument generally.
The microscope depends for its utility and operation upon concave and convex lenses, and the course of the rays of light passing through them. Lenses are usually defined as pieces of glass, or other transparent substances, having their two surfaces so formed that the rays of light, in passing through them, have their direction changed, and are made to converge or diverge from their original parallelism, or to become parallel after converging or diverging.
When a ray of light passes in an oblique direction from one transparent medium to another of a different density, the direction of the ray is changed both on entering and leaving; this influence is the result of the well-known law of refraction, —that a ray of light passing from a rare into a dense medium is refracted towards the perpendicular, and vice versa.
Dr. Arnott remarks: But for this fact, which to many persons might at first appear a subject of regret, as preventing the distinct vision of objects through all transparent media, light could have been of little utility to man. There could have been neither lenses, as now; nor any optical instruments, as telescopes and microscopes, of which the lenses form a part; nor even the eye itself. Rays of light falling perpendicularly upon a surface of glass or other transparent substance, pass through without being bent from the original line of their direction.
Thus, if a ray of light passes perpendicularly to the surface of the piece of glass, it will go on in the right line. But if the same ray of light be directed to the surface obliquely, instead of passing through in a direct line in the direction, it will be refracted in a direction approaching nearer to the perpendicular line, The ray of light is termed the ray incidence, or the incident ray; and the angle which it makes with the perpendicular is called the angle of incidence. That part of the ray through the transparent medium is called the ray of refraction, or the refracted ray; and the angle which it makes with the perpendicular is called the angle of refraction.
The ray of light projected in passing out of the transparent medium, is as much bent from the line of the refracted ray as that was from the line of the original ray. It follows, then, that any ray passing through a transparent medium, whose two surfaces, the one at which the ray enters, and the one at which it passes out, are parallel planes, is first refracted from its original course; but in passing out is bent into a line parallel to, and running in the same direction as the original line, the only difference being, that its course at this stage is shifted a little to one side of that of the original. If from the centre of the circle be described with any radius, the arc measures the angle of incidence, the angle of refraction.