The investigator who desires to see all that his microscope is capable of showing must study the optics of his instrument. The fundamental principles are presented in any good textbook of physics.
In the histological laboratory where preparations are being made the microscope is in constant danger. If it is necessary to use a better instrument for such work, cover the stage with a piece of glass-an old lantern slide is of about the right size-and be extremely careful not to get reagents upon the brass portions.
Everyone who expects to become at all proficient in the use of the microscope should learn to measure microscopic objects and should learn to form some estimate even without measuring, just as one guesses at the size of larger objects. In any measurement one should note the tube length, which is usually 160 mm. Since the use of the nosepiece is universal, it is convenient to have the length measure 160 mm. when the tube is pushed in. Some companies still make the tube so short that it must be pulled out about 15 mm. to reach the length of 160 mm., even when the nosepiece is in place. Where there is no revolving nosepiece, the draw-tube is simply pulled out until the length is 160 mm. Where a nosepiece is used, its height should be measured, and the draw-tube should be pushed in a distance equal to the length of the nosepiece. There are in general use two practical methods of measuring microscopic objects, one by means of the ocular micrometer, and the other by means of camera lucida sketches.
Measuring with the Ocular Micrometer - A stage micrometer and an ocular micrometer are necessary. A stage micrometer should be ruled in tenths and one-hundredths of a millimeter. It does not matter what the spacing in the ocular micrometer may be, except that the lines must be at equal distances from one another. As a matter of fact, the ocular micrometer is generally ruled in tenths of a millimeter, but this ruling is more or less magnified by the lens of the ocular.
Place the stage micrometer upon the stage and the ocular micrometer in the tube, and arrange the two sets of rulings so that the first line in the ocular micrometer will coincide with the first line of the stage micrometer, and then find the value of one space in the ocular micrometer. The method of finding this value is shown in the following case in which the tube length was 160 mm., the ocular a Zeiss ocular micrometer 2, and the objective a Leitz 3. In the ocular micrometer, ninety-eight spaces covered just fifteen of the larger spaces of the stage micrometer. Since the stage micrometer is ruled in tenths and one- hundredths of a millimeter, the fifteen spaces equal 1.5 mm., or 1,500 (.1 Then ninety-eight spaces of the ocular micrometer equal 1,500 ( and one space in the ocular equals 1/98 of 1,500(, or 15.3(, This value being determined, there is no further use for the stage micrometer. To measure the diameter of a pollen grain put the preparation on the stage, using the same objective and ocular micrometer, and note how many spaces a pollen grain covers. If the pollen grain covers five spaces, its diameter is five times 15.3 ( or 76.5 (. In the same way, the value of a space in the ocular when used with the other objectives should be determined. The values for three or four objectives may be written upon an ordinary slide label and pasted upon the base of the microscope for convenient reference.
This method is the best one for measuring spores and for most measurements in taxonomy.
Make a scale for each objective. It is not necessary to make scales for all the oculars, but only for the one in most constant use. It is absolutely necessary to note the tube length, length of the bar of the camera mirror and inclination of the camera mirror, and the level at which the scale is made. A variation in any of these details will change the scale.
In using the stage micrometer, place the cardboard on the- table, and with the aid of the camera lucida sketch the rulings of the micrometer. In Figure 117 note, for example, the scale drawn with Spencer 16 mm. objective, ocular *6. The spaces are drawn from the tenths of a millimeter rulings of the stage micrometer. Therefore each space on the card represents one-tenth of a millimeter or 100 (, and the ten spaces shown on the card represent 1 mm., or 1,000(. By measuring with a metric rule the ten spaces upon the card, it is found that the scale is 102 mm. in length. The magnification of any drawing made with the same ocular and objective, under the same conditions, will therefore be 102 diameters. This does not mean that the magnifying power is 102 diameters, for the magnification of this combination is much less. A scale drawn at the level of the stage would show more nearly the magnifying power of the combination, but would still give too large a figure. The exact size of any object which has been sketched with this combination can now be measured by applying the cardboard scale, just as one would measure gross objects with a rule.
The diameter of the field with this combination is 1,700(. By knowing the diameter of the field with the various combinations, one can guess approximately the size of objects.
Other combinations are made in the same way. An excellent check on the accuracy of the computations is to measure the same object by means of the ocular micrometer and by the scale card. If the results are the same, the computations are correct.
In making sketches, it is a good plan to add the data which would be needed at any time in making measurements; e.g., Spencer objective 16 mm., ocular *6, table, 110, 45, would show that the sketch was made at the level of the table, with the mirror bar at 110, and the camera mirror at an angle of 45.
During a considerable part of the year daylight is often insufficient for successful work with the microscope. Numerous contrivances for artificial illumination have been devised, some of them fairly good, but most of them thoroughly unsatisfactory. More than two hundred years ago Hooke used a device for artificial illumination which probably suggested the apparatus used by the late Professor Strasburger at Bonn. The apparatus in use in our own laboratory is only a slightly modified form of that used in the Bonn laboratory.
The apparatus consists, essentially, of a hollow sphere filled with liquid. A fairly good and practical light can be got with an ordinary lamp by allowing the light to pass through a wash bottle filled with a weak solution of ammonia copper sulphate. A piece of dark paper with a circular hole in it serves as a diaphragm, and at the same time protects the eyes from the direct light of the lamp.
At present, we are using a white 50-watt, 115-volt, nitrogen Mazda bulb, with a shade to protect the eyes. This not only furnishes a strong light, without any glare, but throws a good light on the pencil, which is an important consideration in drawing with a camera lucida.
Optical companies are now making excellent lights for microscopes. These lights furnish good illumination and most of them have the effect of good daylight.
If laboratory tables are small, seating only one student, there should be a plug to attach the table to some convenient outlet; and also another outlet on the table for the microscope lamp. If the table is large, seating four or more students, there should be an outlet on the table for each student, and a single plug by which the whole table may be connected with a convenient outlet.
For elementary classes, which are not likely to use higher powers than a 4mm. objective with an ocular magnifying five or six times, individual lamps are not necessary in a well-lighted laboratory. Half-a-dozen strong lights, of the overhead type, with white shades, serve very well for a class of twenty-five or thirty students.
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