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A Description and Definition of a Microscope.
A Description and Definition of a Microscope
What is a Microscope? The microscope is an apparatus for producing an enlarged image of a small object. How does a Microscope Work? In its complete form it is an elaborate instrument, but to understand its construction it may be looked upon as a complex form of magnifying lens with the addition of means for making delicate adjustments both for moving the lens and the object and for obtaining special forms of illumination. It consists primarily of three parts the body, which carries the observing lenses, the stand or framework, and the illumination apparatus. The body (M) carries an object glass (R), which is attached to The body, the object end by a standard size screw thread, and an eyepiece (0) , which slips loosely into the tube at the eye end in a standard size fitting. It has a telescopic tube, called a drawtube (N), for varying the distance between the object glass and the eyepiece, and a diaphragm (Nl) to prevent reflections from the inner surfaces of the tubes from entering the eye. The body and its lenses combined form the magnifying apparatus. The object to be examined is placed on the stage (D) of the microscope. The object glass if used by itself acts in the same manner as a lantern lens. It throws an enlarged picture of the object to a position (U) at the upper end of the body, just as a lantern lens throws an enlarged picture of a small lantern slide upon a white screen, but instead of its being thrown upon a white screen it is thrown into space. This image is examined with a magnifying lens called the eyepiece (0), by which it is further magnified. If the primary image were projected upon a lantern screen and one were to cut a hole in the screen and stand behind it with a magnifying lens focussed upon the plane of the screen, one would have the same kind of instrument as a microscope on a large and inconvenient scale. The object The object glass (R) in the earliest instruments was a single double convex lens ; it gave an enlarged but very imperfect picture of small objects, the outlines were surrounded by coloured fringes, and the details were fuzzy and indistinct. Such lenses were made several hundred years ago, but in the early part of the nineteenth century is was discovered that the defects of a single lens could be overcome by using several lenses in combination, made of different kinds of glass and of suitable shapes and sizes. Modern object glasses are made of different powers to give different magnifications in the primary image, and, in general, the more an object glass magnifies, the larger the number of lenses that are required to produce a perfect image. Object glasses. The name 2/3, 1/6, or 1/12 inch, as applied to an object glass, representsits focal length. It indicates its ^ ma gnif ying power If an ordinary single distance. Lens of 2-inch focal length is used as a hand magnifying glass, it has to be placed about 2 inches from an object to give a clear image, and the 2/3, 1/6, and 1/12 inch require to be placed at about these respective distances from the object when in use thus the higher the magnifying power of a lens, the closer it must be to the object. Object glasses are not single lenses, but are composed of several, and consequently the focal distance is measureid from a point about half-way between the front and back surfaces of the component lenses. The distance between the foremost lens and the object is, therefore, always considerably less than the true focal distance. This is called the working distance to signify the space between the end of the microscope and the object when it is so adjusted that a clear picture is obtained, or when it is, as it is called, " in focus.".
An enlarged picture of an object placed upon the stage (D) is formed in the neighbourhood of the eyepiece at U, and the eyepiece again magnifies this image, projecting the light into the eye as if it came from an object situated at V. The eye, when placed in a small area (T) through which all Eyepiece. Light passes, and which is known as the eyepoint, sees the final picture of the object as if it were a real object placed at V, 10 inches from the eye. It is assumed for convenience of measurement that this picture virtual is actually 10 inches away, though it may be formed at a somewhat different position according to the adjustment or condition of the observer's eye. Whether the virtual image is actually at 6, 10, or 20 inches is of no importance. It makes no difference to the size of the picture, because when the virtual image is formed farther away it becomes proportionally larger. If E is the eye and 0' 0" are objects of different sizes, they produce the same size pictures in the eye if placed at such distances that they subtend the same angle. The microscope will depend upon the size of this final image formed at V compared with the size of the object being examined. In this connection it should be understood that if a microscope is said to magnify 100 diameters, it means that the picture that is seen is 100 times as long and 100 times as wide as the object would appear if it were taken from the stage and placed in the position V, 10 inches from the eye. In order to express how much larger an object appears when seen through the microscope than when seen by the naked eye, a standard distance must be taken, because an object appears to the naked eye to be of different sizes at different distances. A sixpence is almost invisible at a distance of 100 yards, but it is a large object at 8 inches. Therefore, some standard must be taken for comparison purposes, and 10 inches has been universally adopted. The magnifying power of a microscope always denotes the relative size of the picture compared with that of the original object when placed 10 inches from the eye. If a microscope has a magnifying power of 100, such magnification may be produced by different methods. The object obtaining glass may magnify the object twenty times in the primary image, and the eyepiece increasing the primary image five times will give a total of a hundred. This magnification may also be produced by a lower power object glass which magnifies the object ten times, and a higher power eyepiece which magnifies it again by ten. The same result is obtained as far as magnifying power is concerned, but a different result as regards the quality of the image. Another method of varying the magnifying power is by increasing the distance between the object glass and the eyepiece. To enable this to be done the microscope is supplied with a sliding drawtube (N), which allows the tube length to be varied from 140 to 200 mm. The reason for this increase in magnification is well illustrated by reference to the lantern, in which case the lantern lens gives a larger picture when it projects it upon a screen that is at a greater distance. In the same way the microscope object glass produces a more highly magnified primary image if by slight adjustment in the focussing of the instrument the picture is formed at a greater distance, and the drawtube of the microscope is extended so as to examine the picture formed at this greater distance. The " field of view " is a term applied to the size of the object that can be seen at one time by means of the microscope. To assist in increasing the size of field an eyepiece is made of two lenses instead of a single one. The lower field lens is situated below the position U, where the primary image is produced, and increases the field of view while the upper lens does the magnifying. Suppose that the apparent field of view is a circle of about 8 inches diameter at the position V, where the final image seen through the microscope appears to be. It is evident that with a microscope magnifying 100 diameters, the size of the largest object that can be observed at one time is only 1/100 the size of this field, or about 1/12 inch, so that for this reason alone it is important that a microscope should possess a means of varying the magnifying power. It is sometimes desirable to examine a large area of an object with a small magnifying power, at others a small area with a large magnifying power. The question arises as to whether it is preferable to vary this magnifying power by means of changing the eyepiece, by means of changing the object glass, or by means of lengthening the drawtube. This is influenced by an optical consideration of great importance. In the early days, before it was understood how to correct the errors of a single lens, microscopes were constructed in which the object glass was a single lens, the defects of which were reduced by putting a very small aperture almost a pinhole in front or behind this lens. This meant that only an extremely fine cone of light from each point of the object could enter the instrument. It was soon found that when this was the case, although great magnifying power could be obtained, fine detail could not be seen, but merely a representation on a larger scale of the coarse structure which could readily be seen with a lower magnifying power. Microscope Aperture In order that an advantage should be obtained from the use of higher magnifying power, it was necessary to admit into the microscope a correspondingly larger cone of light from each point of the object, as unless this were done, no advantage could be obtained in the observation of fine details. Such a plan had the further advantage that it collected a larger amount of light and rendered the object more brilliant. The size of the cone of light admitted into the microscope from each point of the object is called the aperture. It is expressed either by the angle of the cone of light entering the microscope or by a figure called the numerical aperture, or N.A. The aperture is of such paramount importance, that the limit Limit of of what can be seen with the microscope does not depend upon JJJSdert what magnifying power can be obtained, but upon what size on aperture. Cone of light can be collected from the object by means of the object glass ; and lenses can be made with a much higher magnifying power, but they cannot be made with a larger aperture, than those in use at the present time. The aperture, therefore, has a direct bearing upon the best method of increasing magnifying power, because, if an object glass can only admit a certain aperture of light, the use of an eyepiece does not alter this property, and therefore to increase the magnifying power by high eyepieces is of no service, when carried beyond that power which is sufficient to enable the detail that can be shown by the aperture of a particular object glass to be seen. The best method of increasing the magnifying power is, Best method therefore, by changing the object glass. Most object glasses have sufficient aperture to allow of the use of an eyepiece of as power. High a power as 15, but, in general, magnification of more than 10 by means of the eyepiece should only be used in special cases, and the object glass should be changed rather than the eyepiece. The same reason makes it undesirable to depend for increased standard magnifying power upon extending the drawtube of the microscope, and the more so in this case because the object glass can only be constructed to work at its best with a particular length of cover glass. Microscope Body To obtain the most perfect results a tube length of 160 mm. Should be used the drawtube of the microscope is graduated, and can be set at this figure. If a revolving nosepiece is in use, this lengthens the body 15 mm., and the drawtube should be set at 145 mm. Instead of 160 mm. ; with a Sloan object glass changer measuring 10 mm. It should be set at 150 mm. Thickness of the thickness of the cover glass used over the object has an immersion lens and but slight influence with the low powers, but is a matter of importance with a highpower dry lens. A 1/6-inch object glass can only be optically correct for one thickness of cover glass, and it is most important to always use those known as No. 1 thickness. The object glasses, unless otherwise ordered, are always made for a thickness of *007 inch (*18 mm.), which is the average thickness of No. 1 cover glass. Thicker cover glasses should only be used for objects to be examined with low powers. The delineation of fine structure depends upon the aperture of the object glass being sufficiently large to produce an image of this fine structure, but combined with this it must possess a sufficient degree of magnification to enable this image to be clearly seen. We may know that the finest lines of an etching or steel engraving exist in a print, but it may be necessary to magnify the image in order to make them visible as single lines to the eye. If the print is magnified further, the fine lines appear thicker, but no further fine lines are there to be seen. Thus lines which are invisible require a certain degree of magnification to see them clearly, but extra magnification beyond this point is useless. So with a microscope object glass, it must possess a large enough aperture to produce the detail in the image, and the magnifying power need not be more than enough to enable the eye to see it clearly. Each object glass has a particular aperture, sufficient to form an image of all the detail that can be seen with the magnifying power given by it in conjunction with a moderate eyepiece.
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How Microscopes Work 
