The Romance of the Microscope / An interesting description of its uses in all branches of science, industry, agriculture, and in the detection of crime, with a short account of its origin, history, and development

CHAPTER III THE ACTION OF LIGHT

It is hardly necessary to remark that the wonderful properties of the microscope depend upon light. Without light, lenses would be useless, objects could not be illuminated and we could not see them. In this short chapter we propose to give a brief outline of the action of light; if our words appear to savour of the school-book, we shall try to avoid it, but, we repeat, if they do so we would remind our readers that the more one knows of the action of light the better use one can make of one’s instrument. As a well-known microscopist has remarked we may be able to afford a costly harp or a costly microscope, but although we may be able to strike a few notes on the former and examine a few objects with the latter, we can only make the best use of either by thoroughly understanding and practising upon it.

The first thing we learn when we study light is that it travels in straight lines. The chief source of light to the inhabitants of this earth is the sun. Now the sun is so far away that, for all practical purposes, the rays of light coming from it may be looked upon as being parallel to one another. That we must always remember, when dealing with the sun, though, of course, it does not apply when we are dealing with lights near at hand, unless they are specially constructed to throw parallel beams or rays, whichever we elect to call them. To prove that light travels in straight lines is not difficult, and we may devise a number of experiments for the purpose. The doors and ventilators of many dark rooms, in which photographic operations are carried on, are constructed on the assumption that light cannot travel round corners. An arrangement as shown in the diagram will allow air, but no light, to pass. If light were capable of going round corners, some other arrangement would have to be devised for the ventilation of dark rooms.

Having learned so much about light, we come to the most important fact of all, as far as the action of light concerns microscopic work. When rays of light travel, from a substance like air into a substance like water, they are bent out of their straight course. Without any desire to introduce a number of unfamiliar words, we may venture to remark that, any substance through which light passes is called a medium. Some media are clearly more dense, more compact or solid—dense is the proper word—than others. Water is more dense than air and glass than either. The bending of light rays is known as refraction. So now we may state our second law a little more concisely, thus:—When light passes from a medium into one more dense, or vice versa, it is refracted, and the more dense the medium into which or from which the light passes the greater the refraction.

A diagram and an experiment should make matters clear. Suppose AB is a ray of light traveling in air and that it falls on a sheet of water, WXYZ, the ray will be bent along BC and its course from air to water may be represented by ABC. Suppose again, WXYZ represents, not water but glass; as glass is more dense than water the course of the ray AB is represented by ABD, it is refracted or bent to a greater extent than the ray which passed from air into water.

For our experiment we need only plunge a stick into water and notice that, owing to this property of light, the stick appears bent, from the point where it comes into contact with the surface of the water.

Some of us may be old enough to remember that once, on either corner of nearly every mantlepiece, there stood an ornament of doubtful utility from which there hung a dozen or more glass prisms. Now the only beauty about these otherwise hideous contraptions was to be seen when light played upon them. Then patches of violet, green, yellow and red were thrown upon neighbouring objects. White light, ordinary sunlight that is to say, is really composed of various colours—violet, indigo, blue, green, yellow, orange and red—which, when combined together, make light as we know it. When white light passes through a prism of glass, it is not only bent out of its course, but broken up into all these colours. A prism, as we all know, when examined at either end, is seen to be triangular in shape. Putting aside for a moment the question of the breaking up of light into its component parts, the path of a ray of light through a prism is shown in the diagram. As the ray passes from air into glass it is bent, because glass is more dense than air; it is bent once more on leaving the prism because air is less dense than glass.

Now lenses are made of various shapes, and those with two outwardly curved surfaces are known as double convex lenses. A double convex lens is usually made with both its surfaces equally curved and in the finer optical work great care is taken to ensure that this is the case. For certain purposes, however, as we shall learn in a moment, one or other of the faces only may be much more curved than its companion and this may be carried to such an extreme that one face is flat, the lens is then known as plano-convex. Lenses may also have inwardly curved faces, if both are of this design they are called double concave; if one face is flat and the other inwardly curved they are known as plano-concave. There are other combinations, for example, one face may be inwardly curved and the other outwardly curved, but the four kinds we have described are all that need trouble us.

It does not require a great amount of imagination to recognise that the double convex lens, that is the lens with two outwardly curved faces is little more than a pair of prisms placed base to base, or more accurately, a number of prisms so arranged as shown in the diagram. Parallel rays of light falling upon such an arrangement of prisms would be bent from their course, as shown by the arrows, and this is just what happens with a double convex lens. Now rays of light from an object, passing through a lens of this shape may follow any one of three courses, according to the position of the object with regard to the lens. In one position and one only the rays after passing through the lens will be parallel to one another, as shown in the diagram.

The only position of the object for the above to take place is when it coincides with a point known as the principal focus of the lens, conversely the parallel rays of light from the sun, after passing through a double convex lens, will come to a point at its principal focus.

Suppose now that the object be placed at a point beyond the principal focus of the lens, the light rays therefrom will, after passing through the lens, converge to a point thus:—

In the diagram O is the object and P the principal focus of the lens.

The third case occurs where the object is nearer to the lens than its principal focus, then the rays after passing through the lens, diverge and never meet.

We have already stated that when white light passes through a prism it is broken up into different coloured rays varying from violet to red. The reason for this is that all the light rays composing white light are not bent equally as they pass from one medium to another. The violet rays are bent the most, the indigo next, blue next, down to red, which is least bent. Once more, considering the double convex lens as made up of a number of prisms, let us represent, by a diagram, the course of parallel rays of white light through it.

A A' represent the parallel rays of white light falling on the lens, L L'. The blue rays are bent more than the red, so the principal focus of the former is at C and of the latter at D. The consequence of this difference in bending of the various coloured light rays would be most serious in microscopic work were not means devised to overcome it. Objects for instance at C in our diagram would appear blue, at D they would appear red, whilst at E E' though no single colour would predominate they would be illumined with many coloured rays, though less strongly than at C or D.

This chromatic aberration, as it is called, depends amongst other things on the nature of the glass used in lens construction. It has been found, however, that a combination of flint and of crown glass will overcome the difficulty. In a later chapter we shall explain the difference between these two kinds of glass. In practice, a plano-concave lens of flint glass is combined with a double convex lens of crown glass and, if the nature of the glass is satisfactory, as also the shapes of the lenses, there is full correction for chromatic aberration, and objects viewed through such a lens will not appear with coloured margins.

There is one further trouble likely to occur in such, or any lens. We write of the rays meeting at a point. In our diagrams we represent the rays by straight lines, really they are much more complicated than they appear in a diagram. It is quite easy to take a ruler and make our imaginary light rays meet at a point, as a matter of fact, where real lenses and real light rays are concerned, it is very difficult, if not impossible, to make the latter meet at a single point. One more diagram may make the matter clear.

Parallel rays A pass through our lens and, as we know, they should all meet at a point P, the principal focus of the lens; the majority do so, but some meet at other points, such as P'. In consequence of this it is difficult to obtain a clear image of an object at P, and the lens is said to suffer from spherical aberration. The perfect simple lens would be one fully corrected for chromatic and spherical aberration.

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