An Elementary Text-book of the Microscope / including a description of the methods of preparing and mounting objects, etc.

by J. W. Griffith

The detection of the finer markings of the Diatomaceæ, which, according to my view, consist of depressions like those upon the valves of Coscinodiscus, is a matter of great difficulty to those who are unaccustomed to the use of the microscope, and who have not a complete set of apparatus. The main point to be attended to in bringing them into view, is to use one-sided oblique light, i. e. to turn the mirror by its stem as much as possible to one side, and then to incline it so as to throw the light upon the object. In this way the valves of the species of Gyrosigma, for instance, appear covered with lines ([Pl. VI.] figs. 16 and 17); but the lines are spurious, i. e. they are the optical expression of rows of minute dots (figs. 16 a, 17 a); and when oblique light is thrown upon the valves from all sides, by means of a special achromatic condenser, in which the central rays are excluded, the dots become distinct, and the markings resemble those on the valves of Coscinodiscus. To show the finer dots clearly, a valve should be crushed, so as to obtain a fragment as flat as possible; for the surface of the valves is curved more or less in all the species. The valves of G. angulatum are generally used to test the quality of the object-glasses of the microscope, and also for practice in “making out” the lines and dots; there are, however, many Diatomaceæ more difficult.

As the nature of these markings is a disputed point, the discussion of which is not adapted for an elementary work, I must refer for further details to the ‘Micrographic Dictionary;’ it may be remarked, however, that some observers have regarded them as cells, and others as elevations or tubercles on the surface of the valves.

The preparation of the valves for showing the markings should be effected by burning the frustules, or the mass containing them, on a strip of platinum-foil over a spirit-lamp. The incinerated mass should then be transferred to a slide, and the valves separated with the greatest care by a bristle mounted in a hair-pencil stick under a low power of the microscope.

This is, however, a substitute for the proper method, which is dangerous in the hands of one unpractised in chemical manipulation. It is this:—The mass of Diatomaceæ (the water containing it having been carefully poured off as far as possible) is put into a Florence oil-flask, and strong nitric acid (aquafortis) gently added, more than sufficient to cover it. The mixture is then carefully boiled over a spirit-lamp for some time. When it is cold, distilled water is added, the whole shaken, and allowed to settle. The watery part is then gently poured off, more water added, and this poured off after settling, and the process repeated until a drop of the water evaporated to dryness on a slide leaves no residue. The Diatomaceæ then form a white sediment at the bottom of the water, and can be transferred to a slide with a dipping-tube. The drop is then dried with a gentle heat, and the valves mounted as dry transparent objects (p. 12).

If the valves have coarse markings, they may be mounted in balsam; but if the markings are fine, balsam makes them much more difficult of detection.

Many of the most beautiful Diatomaceæ are found in the fossil state; and specimens of these are sold already mounted. I would advise those unacquainted with them to purchase a slide of the “Bermuda” or “Richmond” earth, which abounds in the species of Coscinodiscus; and of the “San Fiore deposit,” which contains many species of Epithemia, Navicula, Pinnularia, &c. These may be procured from Mr. Norman, 178 City Road, or from the microscope-makers.

Volvocin´eæ.—The Volvocineæ are inhabitants of clear fresh-water pools, on heaths and bogs. They are very minute, of a rounded or plate-like (tabular) form, of a green colour, and are pretty readily distinguished from most of the other Algæ by their free motion; for they swim about in the water like animals, as which they were formerly considered. They consist usually of groups of thick-walled soft cells, each being furnished with one or two cilia, by means of which the movement of the compound bodies is produced.

In the beautiful Volvox globátor (which is not uncommon) the cells form a hollow sphere ([Pl. VI.] fig. 18), studded with exceedingly minute green spots or zoospore-like bodies, representing the green endochrome of the component cells, and from each of which very fine radiating lines extend, so as to give the surface a netted appearance; the lines consisting of delicate processes of the endochrome, which may be compared with those existing in the cells of the hairs of Tradescantia. In the interior of the parent globes are often seen several young organisms, usually eight, of a deep green colour; these escape by the rupture of the parent, so as to form independent beings. Sometimes they are found of a yellow colour, and furnished with a thick transparent coat; these are called “resting spores,” as they remain for some time before undergoing their full development.

The cilia of Volvox, of which there are two to each of the component cells, are difficult to detect; they are best seen when the organism is dried without a cover, or after moistening them with a little solution of iodine, which dyes them brown.

Synúra volvox ([Pl. VI.] fig. 13) is a still more minute member of this family, and is often found rolling along among Confervæ. The greenish zoospore-like bodies of this Alga have one cilium only, and arise from a common centre by a narrowing of the base (fig. 14).