ON THE DESiUDIE-E OF NORTH WALE-. 41 



only given me the benefit of his large knowledge of this family of 

 plants, but has taken the trouble to examine a large number of the 

 original specimens of Nordstedt, Wittroek, and other Continental 

 botanists, in order to ensure accuracy in the process of identification, 

 and to avoid the possibility of describing as a nev,- species some 

 organism which may have been already recognised as belonging to 

 either the European or extra-European flora. 



I would here also say a few words as to the paramount importance 

 of recording with accuracy the dimensions of microscopic objects, and 

 of delineating those of any particular class to some uniform and con- 

 venient scale. 



Many works on the subject of fresh-water Alga; lose much of their 

 value from defective records of these particulars, and even the admir- 

 able essay of Balis on the British Desmidieae is blemished by the 

 want of uniformity of scale in the beautiful drawings which illustrate 

 it ; hence a tedious process of calculation from the actual dimensions 

 is necessary before a just comparison can be instituted between these 

 several figures, or between these and any specimens under observation. 



A convenient amplification in sketching the Desmidieae is that of 

 l'» i diameters, and by operating on some object of known size it is 

 easy to ascertain, once for all. by what arrangement of the microscope, 

 its objectives and eyepieces, this amplification is obtained in the 

 apparent image formed by the camera lucida, or neutral tint reflector. 

 Thus, with my Eoss microscope, the tube being placed nearly horizon- 

 tally, (10| inches from bottom of eyepiece to surface of paper,) \ inch 

 objective and B eyepieces give an image of an object placed on the 

 stage which is magnified by 400 diameters. A corresponding scale 

 representing l-10th of a millimetre divided into ten parte, each there- 

 fore - -01 mm., being applied to any drawing made in this manner, 

 the actual dimensions of the object which it represents are read off at 

 once. 



This unit. •01 mm., is recognised by Continental botanists under the 

 symbol a, and the dimensions of the Desmidiese are recorded in the 

 following manner: — e.g., -'Long. 21 p. ; Eat. 20 — 21 ft; Lat. 

 isthmi "> ft." It is only in this country that the barbarous nota- 

 tion of inches and their decimal pai-ts still lingers. 



This scale is adopted by Dr. Cooke in the measurement of sporidia 

 and other microscopic organic : ich an equal amplr; I 



conveniently applied. The value of a uniform system < f this kind is 

 self-evident. It enables correspondents at once to institute a just 

 comparison between their specimens without a laborious proce- 

 calculation ; and it is much to be desired that some such arrange- 

 ment, based on the metrical system, should be universally adopted in 

 the descriptions and illust;- pic organisms con- 



tained in scientific works, and in the traii- i all Boci 



devoted to the study and investigation of such subjects. I should 

 rejoice to see the Midland Union of Natural History Societies, and its 



