Half-hours with the microscope; being a popular guide to the use of the microscope as a means of amusement and instruction

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HALF-HOURS WITH THE MICROSCOPE.

-LATE I.

Chapter I. A Half-hour On The Structure Of The Microscope

THE Microscope is often regarded as merely a toy, and only capable of affording a certain amount of amusement. However true this might have been when its manufacture was less perfectly understood, it is now an instrument of so much importance, that scarcely any other can vie with it in the interest of its discoveries. By its means man increases the power of his vision, so that he thus gains a greater knowledge of the nature of all objects by which he is surrounded. What eyes would be to the man who is born blind, the Microscope is to the man who has eyes. It opens a new world to him, and thousands of objects whose form and shape, and even existence, he could only imagine, can now be observed with accuracy. .

Nor is this increase of knowledge without great advantages. Take for instance the study of plants and animals. These beings are endowed with what we call life : they grow and perform certain living functions ; but as to the mode of their growth, and the way in which their functions were performed, little or nothing was known till the Microscope revealed their minute structure, and showed how jtheir, various parts were related to each other. The •Micropeope has thus become a necessary instrument in the hands of the botanist, the physiologist, the zoologist, the anatomist, and the geologist.

Let us, then, endeavour to understand how it is this little instrument has been of so great service in helping on the advancement of science. Its use depends entirely on its assisting the human eye to see — to see more with its aid than it could possibly do without it. This it does in two ways ; first by enabling the human eye to be brought more closely in contact with an object than it otherwise could be, and secondly by magnifying the object looked at.

Just in proportion as we bring our eyes close to objects, do we see more of them. Thus, if we look at a printed bill from the opposite side of a street, we can see only the larger letters ; but if we go nearer we see the smaller letters, till at last we get to a point when we can see no more by getting closer. Now suppose there were letters printed on the bill so small that we could not see them with the naked eye, we could yet, by the aid of a lens — a piece of concave or convex glass, — bring our eyes nearer to the letters, and see. It would depend entirely on the form of the lens, as to how close we could bring our eyes to the print and see ; but this great fact will be observed, that the nearer we can get our eyes to the print, the more we shall see. The most important part of a Microscope, then, consists of a lens, by means of which the eye can be brought nearer to any object, and is thus enabled to see more of it. Magnifying- glasses and Simple Microscopes consist mainly of this one element. In order, however, to enable the eye to get as close as possible to an object, it becomes convenient to use more than one lens in a glass through which we look. When these lenses are fixed in a brass tube, and attached to the Simple Microscope, they are called doublets and triplets. The magnify ing-glasses which are made to be held in the hand, frequently have two or three glasses, by which the power of the instrument may be increased or decreased. Such instruments as these were the first which were employed by microscopic observers ; and it is a proof of the essential nature of this part of the Microscope, that many of our greatest discoveries have been made with the Simple Microscope.

The nearer the glass or lens is brought to an object, so as to enable the eye to see, the more of the object will be seen. So that when we use a glass which enables us to see within one inch of an object, we see much more than if we could bring it within only an inch and a half or two inches. So on, till we come to distances so small as the eighth, sixteenth, or even twentieth of an inch.

Although a great deal may be seen by a common hand-glass, such as may be purchased at an optician's for 7s. 6d. or half a guinea, yet the hand is unsteady, and if they were made with a very short focus, it would be almost impossible to use them. Besides, it is very desirable in examining objects, to have both hands free. On these accounts the glasses, which in such an arrangement are called object-glasses^ are attached to a stand and placed in an arm which moves up and down with rack-work. In this way, the distance of the object from the glass can be regulated with great nicety. Underneath the glass, and attached to the same stand, is a little plate or framework, to hold objects, which are placed on a piece of glass. This is called the stage. Sometimes rack-work is added to this stage, by which the objects can be moved upon it backwards and forwards, without being moved by the hand. Such an arrangement as this is called a B 2

Simple Microscope. Of course many other things may be added to it, to make it more convenient for observation ; but these are its essential parts.

But, although the Simple Microscope embraces the essential conditions of all Microscopes, and has, in the hands of competent observers, done so much for science, it is, nevertheless, going out of fashion, and giving way to the Compound Microscope. Although this instrument is much more complicated, as might be inferred from its name, than the Simple Microscope, it is now constructed with so much accuracy, that it can be used with as great certainty and ease as the Simple Microscope itself. In order to understand its construction, we must study the nature of the lens of which we have been speaking. If we take a lens, and hold it against an object on one side, and place a piece of white paper on the other side, we shall find that, at a certain point, a picture of the object will be produced on the paper. This is the way in which pictures are produced by the camera, of which the photographic artist avails himself for his portraits and sun-drawings. This picture of the object, then, exists in the air at a certain point beyond the lens. Now, this picture may be looked at by another glass, of the same character as the first ; and by this means the object; is brought in a very enlarged form to the eye. Now, this is the principle involved, in the Com^ pound Microscope. In order to effect this object, a tube (generally a brass tube) is fitted to the object-glass, and at the upper part of this tube a glass is fitted on, called an eye-piece. The eye-piece con^ sists generally of two lenses, the object of which is to bring the picture in the tube to a condition in which it can be seen by the eye as readily as it could through the object-glass itself. It is obvious that such an arrangement as this has great advan-

THE MICROSCOPE. O

tages over the simple Microscope. In the first place, by the use of the lenses in the eye-pieces, the object may be magnified to almost any extent. There is, however, a limit to the utility of this magnifying power ; for when objects are greatly magnified, they become indistinct. This is seen in the Oxhydrogen and Solar Microscopes, where the objects are thrown by means of highly magnifying lenses on a white sheet ; and, although made enormously large, their details are much less clear than when looked at by a lens magnifying much less.

Compound Microscopes are generally fitted up with two eye-pieces, one shallow and the other deep. The first has its lenses close together, and magnifies the most, whilst the other has them further apart, and magnifies less. In the use of these eye-pieces, it should never be forgotten that the one which magnifies least is generally the most trustworthy.

The Compound Microscope is now, undoubtedly, one of the most perfect instruments invented and used by man. In the case of all other instruments, the materials with which they are made, and the defects of construction, are drawbacks on their perfect working ; but, in the Compound Microscope, we have an instrument working up to the theory of its construction. It does actually all that could be expected from it, upon a correct theory of the principles upon which it is constructed. Nevertheless, this instrument did not come perfect from its inventor's hands. Its principles were understood by the earlier microscopic observers in the seventeenth and eighteenth centuries, but there were certain drawbacks to its use, which were not overcome till the commencement of the second quarter of the present century.

These drawbacks depended on the nature of the lenses used in its construction. The technical term for the defects alluded to, are chromatic and spherical aberration. Most persons are acquainted with the fact, that when light passes through irregular pieces of cut glass — as the drops of a chandelier — a variety of colours are produced. These colours, when formed by a prism, produce a coloured image called the spectrum. Now, all pieces of glass with irregular surfaces produce, more or less, the colours of the spectrum when light passes through them ; and this is the case with the lenses which are used as object-glasses for Microscopes. In glasses of defective construction, every object looked at through them is coloured by the agency of this property. The greater the number of lenses used in a Microscope, the greater, of course, is the liability to this colouring. This is chromatic aberration ; and the liability to it in the earlier-made Compound Microscopes was so great that it destroyed the value of the instrument for purposes of observation.

Again, the rays of light, when passing through convex lenses, do not fall — when they form a picture — all on the same plane ; and therefore, instead of forming the object as presented, produce a picture of it that is bent and more or less deformed. This is spherical aberration, and a fault which was liable to be increased by the number of glasses, in the same way as chromatic aberration. This also occurred in Compound Microscopes; and the two things operated so greatly to the prejudice of this instrument, that it was seldom or never used.

Gradually, however, means of improvement were discovered. These defects were rectified in telescopes ; and at last a solution of all the difficulties that beset the path of the Microscope-maker was afforded by the discoveries of Mr. Joseph Jackson

Lister, a gentleman engaged in business in London, who, in a paper published in the Philosophical Transactions for 1829, pointed out the way in which the Compound Microscope could be constructed free from chromatic and spherical aberration.

It is this instrument, then, which is most commonly employed at the present day, and to which we are indebted for most of the recent progress in microscopic observation. Its essential parts, in addition to those we have seen in the Simple Microscope, are the tube and the eye-piece. Its advantage over the simple instrument not only consists in its magnifying power, but also in the facility with which it may be employed, in consequence of the more favourable distance the eye of the observer is from the object observed.

In using the Microscope, a great variety of accessory apparatus may be employed to facilitate the various objects which the observer has in view. As this is a book for beginners, we shall only mention a few of these.

Every Microscope is generally supplied with small slips of glass, three inches long and one inch wide. These are intended to place the objects on, which are to be examined. They are either used temporarily or permanently with this object in view, and are called slides. When used temporarily, an object, such as a small insect o» part of an insect, is placed upon the middle of it ; and it may be either placed immediately upon the stage at the proper distance from the object-glass, or a drop of water may be placed on the slide and a . piece of thinner glass placed over the object. This is the most convenient arrangement, as you may then tilt your Microscope without the slide or object falling off.

Objects, when placed under the Microscope, are of two kinds, — either transparent or opaque. When they are opaque, they may either be placed upon the slips of glass, or put between a small pair of forceps which are fixed to the stage of the Microscope, and the light of a window or lamp allowed to fall upon them. This is not, however, sufficient, generally, to examine things with great accuracy ; and an instrument called a condenser is provided for this purpose. It consists merely of a large lens, which is sometimes fixed to the stage, or has a separate stand. Its object is to allow a concentrated ray of light to be thrown on the opaque object whilst under the object-glass of the Microscope. This is called viewing objects by reflected light.

Transparent objects, on the other hand, are viewed with transmitted light : and here again the natural diffused light of day, or of a lamp, is not sufficient ; so that all Microscopes are provided beneath the stage with a mirror, plane on one side and concave on the other. The object of this mirror, which is called the reflector, is to catch the rays of light and concentrate them on the object under the Microscope. The rays of light thus pass through the object, and its parts are seen much more clearly.

Another convenient piece of apparatus is an animalcule-cage. This consists of a little brass box, inverted, to the bottom of which is attached a piece of glass. Over this, again, is placed a lid or cover, with a glass top. The cover can be made to press on the glass beneath, and an object being placed between the two glasses, can be submitted to any amount of pressure thought necessary. This is a very important instrument for examining minute Crustacea, animalcules, zoophytes, and other living

THE MICROSCOPE. V

and moving objects, especially when they live in water.

In the use of the cage and the slide, care must be taken not to break them, by turning the object-glass down upon them. The tube is provided with rack-work and a handle, by which it moves ; so that the object-glass may be adjusted to any distance from the object. It is sometimes a difficult thing, when the object-glass has a focus of not more than a quarter or eighth of an inch, to adjust it to exactly the point at which the object is best seen, by means of the coarse handles on the rack- work ; and the Microscope is often provided with what is called a fine adjustment, by means of which the object-glass is moved down on the object in a much slower and more gradual manner. Care, however, must always be taken lest the object-glass is brought down on to the object or slide, so as to break them. In some cases the lens of the object-glass itself has been broken in this way. Your incautious friends who have never seen a Microscope before, and have sufficient confidence in themselves to attempt to command a man-of-war, though they were in one for the first time, will frequently turn your Microscope up and down with a force sufficient to crack your lens. Such friends should, if possible, be kept from the table till the instrument is ready for them to take a peep, and then it is best to request that they will keep their hands behind them.

The picture of the object brought to the eye in the Compound Microscope is always the wrong end upwards. That is, the picture is always the reverse in the Microscope to what it is with the naked eye. You need constantly to be aware of this, especially if you are going to dissect an object under the Microscope, as your right hand becomes left, and your left right. The observer, however, soon gets accustomed to this, and it creates no difficulty ultimately. But science constantly attends on the Microscope, and ministers to its slightest defects. A little instrument called an erector, composed of a lens which reverses the picture once more, is supplied by the optician, and can be had by those who practise the refinements of microscopic observation.

Another instrument which will be found of considerable service even to the beginner with the Microscope, is a micrometer. This is an instrument for measuring the size of objects observed. Exaggerated notions about the smallness of objects are very prevalent ; and as it is almost impossible to say accurately how small an object is without some means of measuring, a Micrometer becomes essential where accuracy is desired. This is effected by having some object of known size to compare with the object observed. The most convenient instrument of this kind is a glass slide, on which lines are drawn the hundredth and thousandth of an inch apart. If this slide, which is called a stage micrometer, is laid over an object, or the object placed upon it, its relation to the ruled lines will be easily seen, and the size computed accordingly. Many other forms of micrometer have been invented, but this is one of the simplest and most easily used.

It is a good plan to make drawings of all objects examined, or at any rate those which are new to the observer. A note-book should be kept for this purpose, and what cannot at once be identified by the object, may afterwards be so by the drawing. All persons, however, have not the gift of drawing, and for those who need assistance in this way, the camera lucida has been invented. This instrument is applied to the tube of the Microscope when placed at right angles with the stem, in such a way that a person looking into it sees the object directly under his eye, so that he may easily draw its form on a piece of paper placed underneath.

Amongst the accessory apparatus, are various arrangements for concentrating the light on the objects which are placed for examination under the Microscope. Amongst these combinations, is one called the achromatic condenser. This consists of a series of lenses, which are placed between the mirror and the stage, and which may consist of an ordinary object-glass. The stages of the larger kinds of Microscopes are fitted up with a screw or slide, by which the condenser can be fastened beneath and adjusted to the proper focus for throwing light on the object examined. Instruments have also been invented, called illuminators, which are intended to supplement or assist the mirror in throwing light on the object. These are things, however, about which the beginner need not trouble himself. They are amongst the apparatus which contribute to the perfection of the Microscope, but are not amongst its necessary accompaniments.

The same may be said of the polarizing apparatus. The use of polarized light adds greatly to the beautiful appearance of many objects under the Microscope, but it is only in a very few instances in which it can be said to furnish a means of distinguishing one object from the other. It may, therefore, be left to the time when the observer has gone through some little practice with his instrument, or has saved enough money to buy the necessary apparatus.

Having said thus much with regard to apparatus, we will now give some directions for the use of the Microscope under ordinary circumstances. The Microscope may be either used by the light of the sun in the daytime, or at night by some form of artificial light. It is best used by daylight, as artificial light is more likely to tire the eyes.

Having determined to work by daylight, some spot should be selected near a window, out of the direct light of the sun, in which to place a small, firm, steady table. On this the Microscope should be placed, and the object-glass should be screwed on to the tube. The mirror should be then adjusted so as to throw a bright ray of light on to the object-glass. The eye-piece having been previously placed at the top of the tube, the Microscope is now ready to receive a transparent object. If the object to be examined is an animalcule, it may be conveyed to the animalcule-cage by means of a glass tube, which should be dipped into the water where the object is contained, with the finger covered over the upper orifice, so that no air can escape. By taking the finger off when the tube is in the water, the fluid will rush into the tube, and with it the object to be examined. The finger is again applied to the top of the tube, and the fluid obtained conveyed to the animalcule-cage : only such a quantity of the water should be allowed to fall out of the tube on to the cage as will enable the observer to put on the cover of the cage without pressing the fluid out at the sides of the cage. If the water is thus allowed to overflow, it runs over the glasses of the cage, and thus obscures the vision. An object or objects having been thus placed in the cage, it is conveyed to the stage, and placed in such a position that the ray of light passing from the mirror to the object-glass may pass through it. This having been done, the observer must now place his eye over the eye-piece, and use the screw in the tube, and move the object-glass downwards until he gets a clear view of objects moving in the water. This is called focussing. The glass may then be moved up or down, in order that the best view of the object may be obtained. When the object-glass is one of high power, the fine adjustment, which is attached to most compound instruments, may be used for this purpose. When the proper focus is obtained, the object may be moved up or down, right or left, with the hand, or by the aid of the screws which are employed in the various forms of what are called movable stages.

When objects not requiring the live-box or animalcule-cage are to be observed, they may be transferred to the glass slide, by aid of a thin slip of wood, or a porcupine-quill moistened at the end, or by a pair of small forceps. Some transparent objects may be seen without any medium, but generally it is best to place them on the slide with a drop or two of clean water, which may be placed on it with a dipping-tube. When water is used, it will generally be found best to cover the object with a small piece of thin glass. Small square pieces of thin glass are sold at all the opticians' shops for this purpose. The object is then placed under the object-glass as before.

In order to render objects transparent, S£ that they may be viewed by transmitted light, very thin sections of them should be made. This may be effected by means of a very sharp scalpel, or a razor. When objects are too small to be held in the hand to be cut, they may be placed between two pieces of cork, and a section of them made at the same time that the cork is cut through.

Sometimes it is found desirable to unravel an object under the Microscope. If this is the case, only a low power should be used, and the object may be placed on a glass slide, without any glass over, and two needles with small wooden handles employed — ordinary sewing needles, with their eyes stuck in the handle of a hair-pencil, will answer very well. Even when dissection is not to be carried on under the Microscope, a pair of needles of this sort, for tearing minute structures in pieces, will be found very useful.

When opaque objects are to be examined, the light from the mirror may be shut off, and the aid of the bull's-eye condenser called in. The object being secured in the forceps attached to the stage or laid upon a slide, the light is allowed to fall on it through the condenser. The object-glass must be focussed in the same manner as for transparent objects, till the best distance is secured for examining it. The petals of plants, the wings and other parts of insects, with many other objects, can only be examined in this way.

Even the beginner will find it useful to keep by him some little bottles, containing certain chemical re-agents. Thus, a solution of iodine is useful to apply to the tissues of plants, for the purpose of ascertaining the presence of starch. This solution may be made by adding five grains of iodine and five grains of iodide of potassium to an ounce of distilled water. Strong sulphuric acid will be found useful in rendering soft the tissues of both plants and animals ; and in conjunction with iodine it is a test for the presence of cellulose. The strong solution of potash (liquor potassse) can also be employed with advantage in softening and making clear opaque animal and vegetable substances. Nitric acid has even a greater solvent power than sulphuric acid, and may be used for the same purposes. At the same time, the use of these powerful acids requires care, as, if they get on to the object-glass, or it is exposed long to the action of their vapours, its transparency may become impaired.

Chapter II. A Half-hour With The Microscope In The Garden

AMONGST the objects which can be examined by the Microscope, none are more easily obtained than plants. All who have a Microscope may not be fortunate enough to have a garden ; but plants are easily obtained, and even the Londoner has access to an unbounded store in Covent Garden. We will, then, commence our Microscopic studies with plants. On no department of nature has the Microscope thrown more light than on the structure of plants ; and we will endeavour to study these in such a manner as to show the importance of the discoveries that have been made by the aid of this instrument.

If we take, now, a portion of a plant, the thin section of an apple, or a portion of the coloured parts of a flower, or a section of a leaf, and place it, with a little water, on a glass slide under the Microscope, we shall see that these parts are composed of little roundish hollow bodies, sometimes pressed closely together, and sometimes loose, assuming very various shapes. These hollow bodies are called " cells," and we shall find that all parts of plants are built up of cells. Sometimes, however, they have so far lost their cellular shape that we cannot recognize it at all. Nevertheless, all the parts we see are formed out of cells. Cells tolerably round, arid not pressed on each other, may be seen in most pulpy fruits. In fact, with a little care in making a thin section, and placing it under the Microscope, the cellular structure of plants may be observed in all their soft parts.

If, now, we take a thin section from an apple, or other soft fruit, or from a growing bud, or tuberous root, as the turnip, we shall find that many of the cells contain in their interior a " nucleus," or central spot (Fig. 1, PL 1). This nucleus is a point of great importance in the history of the cell, for it has been found that the cell originates with it, and that all cells are either formed from a nucleus of this kind, or by the division of a thin membrane in the interior of the cell, which represents the nucleus, and is called a " primordial utricle."

When the cells of plants have thus originated, they either remain free or only slightly adherent to each other, or they press upon each other, assuming a variety of shapes ; they then form what is called a " tissue." When cells are equally pressed on all sides, they form twelve-sided figures, which, when cut through, present hexagonal spaces (Fig. 2, PL 1). This may be seen in the pith of most plants, more especially the common elder. Transverse slices of the stems of any kind of plant from the garden may be made by a razor, or sharp penknife, and will afford interesting objects for the Microscope.

Cells, during their growth, assume a variety of shapes, and the tissues which they form are n'amed accordingly (Figs. 232, 234, PL 8). Sometimes the cells are very much elongated, or they unite together, to form an elongated tube : the tissue thus formed is called " vascular tissue ;" but where the cells retain their primitive form, it is called " cellular tissue." A very interesting form of the latter is the " stellate" tissue found in most water-plants, and especially regularly developed in the common rush (Fig. 3, c