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Information on Microscope Glass Cover Slides.
Microscope Cover Glass
What is Cover Glass? Cover glass is a specially thin form of glass prepared for use with the microscope. It is made in squares or circles of 5/8 to 7/8 inch diameter, or can be cut to any particular size required. It is made in three thicknesses however the average thickness is . '006 in. Or 15 mm. The thickness of cover glasses varies about 20 per cent, in different individual measuring pieces, and absolute uniformity of thickness can only be obtained cover glass by selection. A screw micrometer is the most useful form of appliance for measuring cover glasses. Cover glass can also be measured by the microscope itself, the fine adjustment milled head of a microscope is provided with a series of divisions, and the amount that the body tube of the microscope is moved bj; the revolution of the milled head for one division is known. A high-power dry object glass should be used, and the cover glass to be measured placed under the microscope, resting on a glass slip so that one edge of the cover glass appears near the centre of the field of view. The microscope should now be carefully focussed on to specks of dust on the upper surface of the cover glass and the position of the fine adjustment milled head observed. The milled head provided with the divisions should then be turned till the dust on the slip is in focus and the number of divisions that the milled head has moved to make the alteration noted. This number multiplied by the value of one division gives the thickness of the cover glass. It is necessary to focus particles of dust which are situated on the slip to one side of the cover glass, and not seen through it, as the optical path seen through glass is not the same as that in air. If it is desired to ascertain the thickness of a cover glass of measuring a mounted specimen where the edge of the cover glass cannot JJJ^Juw* be observed, the microscope may be focussed to the dust on the of mounted surface of the cover glass and then to the object itself, but the result so obtained will be too small, and must have one-half as much added to it. If the motion of the adjustment is ten divisions, the true thickness is 15. Before Using Cover Glass Slides It is essential that cover glasses before use should be thoroughly cleansed, and all specks, hairs, and fibres be removed. In most cases a little soapy water will remove all dirt and grease, after which they should be rinsed in clean water and dried with a clean linen duster or chamois leather. Some microscopists use two flat boards covered with chamois leather, between which the cover glasses are rubbed, reversing the glass during the process to make sure that it does not adhere to one board, thus cleaning only one side. With low-power object glasses 1 inch (32 mm.), 2/3 inch (16 mm.), 1/3 inch (8 mm.) The thickness of cover glass used is of little importance ; but for high powers 1/6 inch (4 mm.) Or higher power dry lenses it is most important to always use the thinnest covers (No. 1), because with high-power object glasses which are not immersion lenses a variation in the thickness of the cover glass affects the correction of the object glass. An object glass can only give the most perfect image when used with a cover glass of a particular thickness, and they are always adjusted for the No. 1 cover glass. The 1/6-inch (4-mm.) Object glass is very sensitive in this respect, and one apochromatic lens of this power is provided with a correction collar to adjust for cover glass of different thicknesses. As microscopic cover glass is sold by weight, the cost of the No. 1 glass is not materially more than the No. 2 or 3, because a larger number go to the ounce. If a specimen in the nature of a leaf, a fibre, or powder, is to be examined under a high power, it is best to place such a specimen on a glass slip and place a cover glass over it to flatten it out and hold it in position, preferably in a drop of water. In this case a slip with a ledge against which the cover glass may rest is a convenience. If the specimen is to be examined in fluid, a drop should be placed on the slip and a cover glass put down over it at an angle in such a manner that the cover glass touches one side of the drop first, and is then allowed to gradually fall so as to prevent air bubbles being enclosed (Fig. 49). Blood films, or specimens of bacteria which are to be examined and then destroyed, may be dried by heating over a spirit lamp upon the slip or cover glass. If they are to be examined with a dry object glass, they should be dried upon the cover glass and placed film downwards upon the slip. If an immersion object glass is used they may be dried on to the slip and the use of a cover glass dispensed with, for the whole space between the object and the lens is filled with what corresponds to glass. The thickness of the cover glass, therefore, makes no difference optically, but unless the object is thoroughly dried a cover glass may be required to prevent the object from floating off into the immersion fluid. By putting a drop of Canada balsam between the cover glass and the slip, and firmly pressing them together, a permanent mount may be prepared. When objects in a drop of fresh or salt water are placed between examination a cover glass and a slip, the superfluous fluid around the cover glass should be removed with blotting or filter paper, and capillary attraction will hold the cover glass in position when the slide is placed at an angle. If a specimen is to be examined for a long period, a piece of cotton may be placed between the cover glass and the slip, one end of which dips into a bottle or capsule of water at a higher level than the slip, and the other in a similar bottle at a lower level. By this means the slide will be kept moist and objects can be kept alive for a considerable period. Small organisms, such as infusoria, bacteria, or protozoa, have sufficient room in the thin layer of water between the cover ho glass and the slip to live and move freely, but larger objects, such as rotifers, entomostraca, etc., require more room. For use with such objects, slips are made with ties, and are known as slips withHollows. They are used in the same way as ordinary slips, the water which fills the cavity holding the cover glass in position by capillary attraction. When such objects are in fluid, the removal of the superfluous water is sufficient to make the cover glass adhere to the cell. If insects are to be examined dry, the cover glass may be made to adhere to the cells by placing a Slip with Cell smear of grease or vaseline around the cover of the upper edge.
For the examination of aquatic weeds, algse, and animalcula Trough, with low powers, a trough is a useful apparatus. Examples: Convenient form mounted on a 3 X 1 slip ; it has usually a space for water about 2 mm. Thick. It is made with an upper glass either of thin microscope cover glass, about '25 mm. Thick, or a thicker glass about 1 mm. Thick. A glass trough cemented together. This has dimensions of 1 X 1 X 1 inches, and is made of glass about 1 mm. Thick. It is only suitable, owing to the thickness of the glass, for use with low powers. No. 3413, Slip with Trough. A very useful form of trough, known as Beck's glass trough, is made of a 3 X 1 glass plate, into which are fixed two screws and milled nuts, each holding a clamping plate. A half-circle of indiarubber made from an elastic band is laid on the 3x1 slip, and a glass cover plate of any required thickness is placed on the top. The whole is clamped together by the milled nuts. As all the parts take to pieces, it can be readily cleaned, and cover glasses or separating bands of any thickness can be used. Separating bands of the very thinnest material, such as dental rubber, or even paper, can be used, so that the layer of material being examined is exceedingly thin. This is of great convenience when it is desirable to examine the specimens by dark-ground illumination or with high powers. It is a very convenient appliance also for the examination of aquatic specimens. These can be first arranged in position on the lower 3 X 1-inch slip within the area surrounded by the rubber band, the cover may then be placed in position and sufficient fluid dropped in. If a small circular cover glass be cemented in the centre of the 3 X 1-inch lower glass, a small drop of fluid can be confined to the centre of the field for examination. It can be used with substage condensers or darkground illuminators. A live box consists of a plate 3x1 inches, with an aperture in the centre of which is fixed a short brass tube carrying at the top a glass plate. Over this tube slides a cap, in the top of which a cover glass is held by a screwed cell. The object to be examined is held between the two glasses. This appliance is useful for examining living insects or for flattening out thin, uneven objects, such as a piece of a leaf or fabric. It is chiefly used with low powers, as substage illuminating apparatus cannot be readily used. A form of live box known as the Rousselet live box is useful for high powers. The principle is that . No. 3420, Live Box. Of an ordinary live box, but the fixed lower glass plate is on the level of the stage, and a substage condenser or high-power illuminator can be used with this live box. When a very small object is to be examined, a still smaller cover glass may be cemented with Canada balsam to the centre of the lower glass plate, and the object is thus confined to the centre of the field. It is 2 X 1 inches, and is not suitable for use on a mechanical stage. The Beck compressor is a 3 X 1-inch plate of glass at one end compressor, of which a circular pillar is fixed. This pillar carries an arm which holds a thin cover glass 3/4 x If inches. The arm is raised or lowered by a screw at the top of the pillar, which mechanically varies the space for holding the specimen. The arm carrying the thin glass can be swung to one side for placing the specimen in position and then lowered to the required amount. For many purposes this compressor is more convenient than a live box, for by means of the delicate screw motion a living object may be held stationary without being crushed. Also, the slip being made of glass, it can be kept clean, and the thin glass which is attached to the arm by spring clips can be readily removed for cleaning, or replaced if broken. It can be used with substage condensers and dark-ground illuminators. A convenient method of holding small solid objects for stage observation under the microscope is by means of a pair of stage ^^P 8 forceps, which are attached to a 3 X 1-inch ebonite plate. The plate is either held by the mechanical stage or, if the microscope is not fitted with the latter, by means of the spring stage clips. On the plate is a metal fitting holding a rod, which has at one end a small pair of spring forceps opened by pressing the two pins together, while at the other end is a cork into which specimens may be pinned. The forceps can be unscrewed from the rod, which can then be reversed in its fitting so that either the forceps or the cork can be brought into the centre of the field ; they can also be rotated so that all parts of the object they hold can be examined. This apparatus is useful for the examination of small insects, botanical specimens, fragments of rock, tissues, and other small solid bodies. A case can be supplied containing apparatus for holding objects, which includes 3 X 1-inch slips, a slip with ledge, a slip with hollow, a trough on slip, a Beck glass trough, a live box, a Beck compressor, stage forceps, and a supply of thin glass. The Rousselet live box is not included, as it is not of the standard 3 X 1-inch size. Mounting permanent specimens for the microscope is a subject that is beyond the scope of this book. The microscopist should be provided with a bottle of Canada balsam dissolved in benzol or xylol, which is a transparent cement, and a bottle of glue, which is a brown shellac cement. Many objects can be mounted by means of these two cements. Small shells, botanical and entomological specimens, diatoms, and other small objects may be attached to a 3 X 1-inch slip with gum or Canada balsam inside a cell of paper, vulcanite, or glass of a thickness sufficient to protect them, and with a cover glass cemented to the cell. A narrow ring of cement of the diameter of the cover glass, dried upon the slip, is often sufficiently thick to protect small objects when a cover glass is cemented to the surface of this ring. Microscope Turntables A turntable is an appliance for making rings of cement on a 3 X 1-inch glass slip and for placing a protecting ring of cement round a circular cover glass, or cell after it has been cemented on. The slip is held on to the circular revolving table by spring clips, and by holding a camel's-hair brush, which has been dipped into cement, against the slip as this table spins round, a layer of cement is left ring. Many objects can be placed on a slip and a drop of Canada balsam dropped upon them, a cover glass being then placed over the drop before it has set. The specimen is thus permanently preserved. This is all that is required with such specimens as dried blood films or stained bacteria. It is essential, however, that such specimens should not be moist, as water will not mix with Canada balsam, and some objects require to be first soaked in absolute alcohol or turpentine to remove the water or air.
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Microscope Equipment 
