192 FRESHWATER ALG.E. 



It is woi'tli a<l(liii<^ tliat the Desniids 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 clean and free from dirt. 



The methods of mounting the various classes of Algaa 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 pi-eserved for indefinite periods. 



A few words, in conclusion, on the subject of recording obsei-vationa 

 made under the microscope. All Algaj, 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 iu 

 pencil, pen and ink, or other material, made from the livi)i(i 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 a 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 arrange- 

 ment ; 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 mJirit 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 Ilassall's "Freshwater Alga-s' 

 the only systematic work upon this subject in the English language, are 

 wholly useless from the absence of all dimensions, 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 bo in all cases recorded in fractions 

 of an inch or in millimetres ; and the following method of ascertaining 

 them at a glance will bo 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 l-50th of an inch. A glass slip ruled into thousandths of 

 an inch is placed on the stage, and each of the objectives in oz-dinary use, 

 say from ^ to 2 inch, being screwed into its place in turn, it is only 

 necessary to observe which divisions of the ridcd eye piece correspond 

 with the actual thousandth-hnes iu the slip to obtain a gauge of the 

 dime"yious of an object corresponding to ouch division of the c\c piece. 



