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Information about the optical portion of the microscope.
Optical Portion of the Microscope.
The optical portion of the ordinary microscope includes the eye-piece, object-glass, and the mirror from which the light is reflected so as to pass through the object. The image in the compound microscope is inverted, but this incon venience may be obviated by causing it before it reaches the eye to pass through another set of lenses inserted in the tube of the microscope, and termed the erector. This instrument consists of a tube, at one end of which is a planoconvex lens, and at the other a meniscus, a diaphragm being placed about midway. This is inserted in the tube of the microscope above the object glass, and, like the similar arrangement in the telescope, reverses the image. Negative Eye-piece: The eye-piece in ordinary use is the negative or Hughenian eye-piece. It consists of two plano-convex glasses, the flat surfaces of each being directed upwards. The one nearest the eye of the observer is the eye-glass, and the other at the greater distance the field-glass. The large microscopes are usually supplied with two or three eye-pieces, so that the amplification of an object may be doubled or trebled. Reiner's eye-piece is made like the above, but the eye-glass is an achromatic combination.
The Positive Eye-piece: Is only used in those cases in which it is necessary to see distinctly some object in the eye-piece, as, for instance, an instrument for measuring, at the same time that the object is in focus. In this eye-piece the convex surfaces of each of the two glasses are directed towards one another. Object-glasses: The object-glasses, used in the best instruments, are of English manufacture, but many really good object glasses are now furnished with the cheap microscopes, some of which are made on the Continent, and are produced at a very cheap rate. The two most useful object glasses for a student are the quarter of an inch which, with the No. I eye-piece, should magnify from 200 to 220 diameters, and the inch which should magnify from 30 to 40 diameters. The definition of these glasses should be good, and they should transmit plenty of light. Any lines in a structure examined by them should appear sharp and distinct. The field should be flat, every part of it in focus at the same time, not too small, and there should be no coloured rings round any object subjected to examination. The achromatic object glasses consist of three sets of lenses, each of which is in itself compound. Many of the new French and German objectives have excellent defining power, and are produced at very small cost. Of late Mr. Swift and other English makers have succeeded in making objectives quite equal to them, at the same price. By employing a glass of great refractive power Messrs. Parkes and Son, of Birmingham, have made some high power lenses, which have excellent defining power, at a very low price. The one-sixth cost, and the one-seventh, of 105 aperture. The working distance of these objectives from the thin glass covering the object is greater than in the case of most objectives of the same degree of magnifying power.
Object-glasses of high power are now generally made so that the object must be viewed through a thin stratum of distilled water placed between and touching the surfaces of the front lens of the objective and the covering glass (a immersioii). The image has a peculiar brightness, and the object is more highly illuminated, because more oblique rays are admitted than would otherwise pass into the lens; the working distance of the objective is somewhat increased, while the price of glasses of the same magnifying power is less. Immersion object glasses were first made by M. Hartnack, of Paris, the successor of Oberhauser, but they are now produced by all the best makers. Mr. Stephenson uses oil instead of water, and thus gains great advantages in resolving P. Angulatum, and such objects. Zeiss has lately made some improved immersion lenses, for use with oil of cedar- wood. Spherical and Chromatic Aberration: Unless the objective is properly corrected for spherical and chromatic aberration, it is valueless to the observer. Spherical aberration may be known by the want of sharpness when a fine line or small spot, or body with a well defined circular outline, is examined. The lines seem to be blurred and foggy, and when there are several lines or spots near to one another, they appear to run together, producing a general shadow, instead of each one being distinctly defined and separated from its neighbours. If the glass has not been properly corrected for chromatic aberration, lines and dots are seen with coloured fringes, blue if the lens is under corrected, reddish if over corrected. Flatness of Field: Can be tested by moving an object from one part of the field to another without altering its distance from the object glass. If the field is flat, the object will appear equally well defined in all parts, but if the glass is defective in this particular, an object accurately focussed in the centre will be found to be blurred and out of focus when it is moved to the circumference. Or a stage micrometer, 60, ruled to hundredths and thousandths of an inch, may be brought into focus. If the lines are sharp and clear, and perfectly parallel to one another in every part of the field, the glass is a good one; but if some appear curved and thicker at the circumference of the field than at its centre, the glass is defective. It is not to be supposed that, even if the most minute directions were given, the student just commencing work would be able to test the object glasses he was about to purchase, in all necessary particulars. Generally he must trust the maker, but if he desires to ascertain if his object glass is good, perhaps the simplest plan is to compare the images produced by the same object first placed under his own power and then under a glass magnifying in the same degree, but of known excellence. Angle of Aperture: For ordinary work it will be found inconvenient if the object glass, when in focus, comes too close to the object. This is a defect in glasses having a high angle of aperture. The angle of aperture is the angle made by two lines. From opposite sides of the aperture of the object glass with the point of focus of the lens. Glasses with a high angle of aperture admit much light, and define many structures of an exceedingly delicate nature, which look confused when examined by ordinary powers. For general microscopical work, however, glasses of medium angular aperture are to be recommended. Glasses having an angle of 150 degrees and upwards are valuable for investigations upon many very delicate and thin structures, such as the diatomaceae; but such powers are not well adapted for ordinary work. The importance of arranging the object very carefully and the necessity of paying great attention to the adjustment and illumination, render these glasses inconvenient for general observation. The penetrating power of glasses with a low angle is greater than in those of a high angle of aperture, and exact adjustment of all the lenses is of the utmost importance. If then the object to be examined is of considerable thickness, an objective of moderate angle is to be preferred, but this view can only be acted upon within certain moderate limits, for if the angle is reduced too much there is great loss of definition. The refraction produced by the passage of the light through the thin glass covering the object varies according to its thickness, and it has been found necessary to render the higher objectives capable of being adapted to this varying refraction. An arrangement for "correction" is especially necessary in the case of glasses of high angle of aperture, and usually consists of a screw collar, by turning which the distance between the front and second pair of glasses may be increased or reduced. An engraved line shows the point to which the lens should be set for uncovered objects. Its adjustment for covered objects is to be effected in the following manner: arrange the objective as if for an uncovered object ; then any object covered with thin glass is brought into focus by moving the body of the microscope; next the milled adjustment ring adapted to the object-glass is turned round until any particles of dust upon the upper surface of the thin glass covering the object are brought into focus. The lens is thus "corrected" for the thickness of the cover, and it only remains to re-focus the object. The mechanical arrangement usually employed in this country for "correcting" is not quite satisfactory, especially in the case of the very high objectives. The screw works too hard, and the thread is too coarse. Mr. Wenham has introduced a great improvement, which entirely overcomes these objections, and enables the observer to "correct" from time to time while he is examining the object. The middle and posterior lenses are made to alter their position instead of the front lens. This is a very valuable improvement. Other modifications have since been made, among the most simple and advantageous of which may be mentioned the very ingenious movement. The screw should work so easily that the observer may turn it first in one direction and then in the opposite while the object is being examined. In this way he will be able to ascertain the exact point at which a particular object is seen in the greatest perfection. In using the highest magnifying powers different objects in the very same preparation will vary in clearness according to the depth at which they lie in the preservative medium, and the lens will accordingly require varying adjustment. The Mirror, should slide upon a perpendicular bar or tube beneath the stage, so that it may be arranged near to, or at a distance from, the object, and it should be capable of being inclined at any angle, so that rays of light may be reflected from it and made to pass directly through the object, or thrown upon it very obliquely. The mirror should be of at least two inches in diameter, one surface quite plane and the other concave, so that a strong light may be condensed upon the object, if desired. The achromatic condenser and other pieces of apparatus of advantage for examining objects by transmitted and reflected light.
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