It is worth adding that the Desmids are easily separated from other plants by shaking the gathering up in a tube and pouring the whole into a watch glass, when, from their higher specific gravity, they sink to the bottom, and, by a little careful manipulation, may generally be recovered quite lean and free from dirt.
The methods of mounting the various classes of Algæ we reserve for discussion in a subsequent paper. For the present let it suffice to state that, by attention to certain indispensable details of manipulation, these plants may be preserved for indefinite periods.
A few words, in conclusion, on the subject of recording observations made under the microscope. All Algæ, however carefully mounted, do, in course of time, more or less lose their exact form and natural appearance. Hence it is most desirable to supplement their collection by sketches in pencil, pen and ink, or other material, made from the living plant. Various ways of doing this are recommended, but we have found the use of what is known as a neutral-tint reflector, to be by far the easiest and most effectual. This apparatus consists simply of e piece of the thinnest possible microscopic glass, fitted into a cap, which replaces that of the ordinary eye-piece, and holds the glass-plate at an angle of 45° to the axis of the instrument. It is, in fact, a miniature Pepper's ghost arrangement; and the microscope being clamped in a horizontal position and focused, upon looking down upon the glass-slip the observer sees the image of the object reflected to the eye, but apparently at a distance below the reflector equal to that between it and the object itself. By placing a sheet of paper on the table underneath, and adjusting the relative illumination of the object and the paper, so that the point of a pencil is clearly seen on the latter, it becomes easy, with a few hours' practice, to trace the smallest details, In practice, it is best, however, to obtain an accurate outline in this way, and the spirit of the object is better given by filling in details from direct vision in the microscope.
This method gives a drawing perfectly accurate, and to a scale easily ascertained.
Many of the descriptions given in Hassall's "Freshwater Algæ," the only systematic work upon this subject in the English language, are wholly useless from the absence of all discussions, and it is impossible to decide whether the species described are really distinct ones or are needlessly multiplied, by reason of this vital defect. It is of primary importance that dimensions should be in all cases recorded in fractions of an inch or in millimetres; and the following method of ascertaining than at a glance will be found simple and satisfactory.
A plain circle of strong glass may be obtained from any optician, of size to drop into the focal point of the eye piece, and ruled into squares, conveniently of 1-50th of an inch. A glass slip ruled into thousandths of an inch is placed on the stage, and each of the objectives in ordinary use, say from ¼ to 2 inch, being screwed into its place in turn, it is only necessary to observe which divisions of the ruled eye piece correspond with the actual thousandth-lines in the slip to obtain a gauge of the dimensions of an object corresponding to each division of the eye pieces.
