PREPARATION OF DESMIDS. 133 



the need for a suitable immersion liquid. Dr. Van Heurck has 

 used this new lens for some weeks, and speaks very highly of its 

 resolving and illuminating power. He has also managed to apply 

 it to some of the bacteria. The focal length is i-ioth inch, and 

 the mounting medium has an index of 2*4. In oblique light, 

 Amphipleura pelhicida is entirely resolved in beads, and it is seen 

 to have 3,600 transverse, and 5,000 longitudinal striae per milli- 

 metre. No wonder these beads have hitherto been so difficult to 

 resolve. Only three of these wonderful objectives have been 

 made, and as they cost ;£'4oo each, it is not likely there will be a 

 great demand for them. But as they are in the hands of a few 

 experts, we may hope for the eventual solution of a few micro- 

 scopical problems. 



The Preparation of Desmids. — We have received a 

 valuable communication from Mr. W. H. Walmsley, of Phila- 

 delphia, on this subject. We are thankful to Mr. Walmsley for 

 his kindly and appreciatory remarks on the Article in our last No. 

 on Desmids mid Volvox. As this gentleman has spent many years 

 on this kind of work the following extract from his letter may be 

 of service to some of our readers : — 



" Having been perfectly successful in preserving the colour in 

 many of our fresh-water Algae, it may be that the same method 

 would prove successful with Desmids. My plan is simply to have 

 a wide-mouthed bottle, with glass stopper, filled with distilled water, 

 in which I have a number of pieces of Camphor. When it is 

 desired to mount the Algae, I place a portion of the same in some 

 of this camphorated water, to which a few drops of glycerine have 

 been added, in a watch glass. At first it will become a yellow 

 lemon colour, but after a few hours the original green returns in 

 its full vividness, and then I at once mount in a shallow cell, with 

 a portion of fluid. Drapariialdia pliimosa thus mounted twenty 

 years ago, is to-day as beautifully green as at first, and the chloro- 

 phyll seems to be unchanged." 



Hantzch, as we said, uses alcohol instead of camphor, in order 

 to get the proper specific gravity for the medium, and so prevent 

 contraction of the cell. 



