The slides may be treated in the same way, or they may be easily prepared by immersion in a solution of washing soda, and then washed and dried. This process may be used in cleaning the balsam or styrax from old slides.
Preparation of Strewn Mounts.—Place several covers on the mounting stand. With a dipping tube, cover each circle with distilled water, and add a small drop of the prepared diatoms, being careful to avoid any vibration of the stand. Heat the stand until small bubbles begin to appear, remove the lamp, and allow the water to evaporate. If the above method is carefully followed, the diatoms will be deposited in an even layer, provided the material is not too dense. Take a slide, centre it, and place a small amount of styrax on the centre. Invert the prepared cover, and gently place it upon the styrax. Heat the slide on the mounting stand until the styrax bubbles and then allow to cool. If bubbles still remain, heat again until they disappear. It is well to mount several slides more than required, as some may be imperfect.
Preparation of Selected Mounts.—Take a slide, place a minute quantity of beeswax on two places at a distance apart nearly equal to the diameter of the cover used. Place a cover on the wax and press it down flat, or sufficiently to keep it in position. Dip a fine needle into the following cement:
| Glacial acetic acid | 12 drachms |
| Gelatine | 02 drachms |
| Alcohol | 01 drachm |
This is made by adding the acid to the gelatine in a water-bath and then the alcohol, and filtering. Apply the moistened needle to the centre of the cover and spread as small a quantity as possible in a thin layer. Now place the slide upon the turn table, centre it with respect to the position of the gelatine, and with the finest sable brush draw a circle about a tenth of an inch in diameter around the gelatine in water-color (Windsor), blue or vermilion, or in India ink. Instead of the water-color, a circle of tin-foil the size of the cover and pierced with a hole in the centre may be used, but the colored circle is to be preferred, as, when brought into view, it indicates exactly the focus required for observing the diatom.
The bottle containing the cleaned material, which has been kept in water and alcohol, should be refilled with distilled water and well shaken, when a small portion may be taken up with a dipping tube and evenly distributed over a portion of a slide and then dried. By the use of a mechanical finger, fitted with a small piece of finely spun glass attached by wax to the holder of the finger, when the microscope is focussed until the glass thread touches the diatom selected, it will adhere to the thread. Raise the body of the microscope, remove the slide containing the spread material, or move it to another part of the stage, and place the slide with the prepared cover in the same position. Now carefully lower the body-tube of the instrument until the diatom rests upon the gelatine, breathe gently upon it, remove the cover from the slide, invert it over another slide containing a drop of styrax and proceed by heating to mount as before. The size of the diatom, the amount of gelatine, and several other factors, will enter into the question of success or failure. I have, however, employed the above method and have mounted thousands of slides of selected diatoms successfully. It is necessary to avoid any air current which will cause the diatom to fall from the thread. On very cold days the glass thread sometimes becomes electrified and the diatoms will not stick; on sultry days in August in our locality the diatoms will stick too closely.
By the same method, slides of arranged diatoms can be made using a glass circle properly marked with lines in the eye-piece. Care should be taken to use glass threads more or less in proportion to the size of the diatoms. A cat's whisker is preferred by some to the glass thread. It has the advantage of not breaking, but unless it is quite short it is too flexible. If the point of the thread becomes covered with gelatine, lower it into a minute drop of water upon a separate slide, and by moving it about it will be cleaned. The diatom itself may be washed in the same way, if it is not too small.
Instruments Required.—For collecting, in order to determine the quality of the find, any simple lens of fifteen to twenty diameters is sufficient. A Stanhope is quite useful although difficult to obtain, while an achromatic triplet of sufficient power will probably be all that is necessary. For selecting with the mechanical finger, an objective of two-thirds-inch focus is the most convenient, but for determining species a one-fifth-inch is needed, an immersion objective being essential for minute forms.
No particular form of microscope is required. Any instrument having standard parts, inclination of the body to the axis, a sub-stage condenser and movable stage, will prove serviceable in nearly all investigations. For critical work, measurement of striæ and location of specimens on the slide, the large models of Bausch and Lomb leave nothing to be desired. One smaller instrument may be used for rapid examination and for selection with the mechanical finger. If the stage is supplied with a vernier, the diatoms can be located rapidly and recorded for future reference. The Zentmayer Army Hospital stand with mechanical stage is excellent. The Continental stands, convenient for laboratory work, especially in the examination of bacteria, are not so serviceable as the larger stands of American and English make. The stand especially designed by Dr. Henri Van Heurck, the celebrated Belgian naturalist, is, without doubt, admirably suited to the investigation of the Diatomaceæ. In the form of the Circuit Stage as made by Watson and Sons, of London, supplied with proper condenser and mechanical stage with vernier attachment, it has been used in the preparation of the present work with much satisfaction.
The drawings have all been made with an Abbé camera lucida, a 3 mm. objective and a No. 10 eye-piece, producing a magnification of about 800 diameters. All illustrations are from actual specimens in my cabinet or, in a few instances, from slides sent me by friends. In the measurement of striæ and puncta, the number in ten microns is stated, and will be found to be approximately correct in most of the drawings, except when the number is in excess of twenty in ten microns, in which case it is impossible to represent the markings accurately on figures of the magnification adopted. All drawings are from specimens in this locality, except in a few cases mentioned in the text.