1887.] NEW-YORK MICROSCOPICAL SOCIETY. 71 



" Medium pow.ers, ^, /^, low-angled i in. ; minute hairs on 

 proboscis of Blow-fly; hair of Pencil-tail {Folyxenus lagaries); 

 diatoms on a dark ground. 



"Medium powers, wide angled, \, ^, i in.; P.formosum and 

 N. lyra in balsam or styrax ; bacteria and micrococci stained. 

 'I [ " High powers, wide angled immersions ; the secondary struc- 

 ture of diatoms, especially the fracture through them. Navicula 

 rho77iboides in balsam or styrax, bacteria and micrococci stained. 



" Chromatic aberration is not so important as the spherical, be- 

 cause some very fine object-glasses have a great deal of out- 

 standing color. 



" Tests for low powers ; thin sections of deal, the coarse struc- 

 ture. Medium powers ; the discs in ditto. High powers ; 

 Podura scale and P. formosuvi. So it will be seen that there is 

 only one point of paramount importance in an objective to be 

 tested, and that is its spherical aberration. The other qualities 

 can be measured." 



■ Mounting Diatoms in Situ. — The following correspond- 

 ence, published ten years ago, will well bear repetition, on account 

 of the excellence of the methods advocated, and may perhaps, 

 be a valuable reminder to some one looking for advice. " It is 

 often desirable to mount diatoms in situ, as they grow attached 

 to algae or other aquatic plants, either to illustrate their mode 

 of growth, or to obtain them when in too small quantity for any 

 of the processes of separating or cleaning. 



" I have never found any method of mounting satisfactory until 

 I tried the following, which with all the algae that I have tested, 

 gives satisfactory results. 



" The algae are thoroughly dried, as usually stuck on paper. It 

 is presupposed that all extraneous dirt has been removed. I 

 have provided a slide with a circle of ink, marking the center on 

 the reverse side, after the plan of my friend, Prof. C. Johnston, 

 cover glass, a bottle of Canada balsam solution in chloroform, a 

 bottle of chloroform and a watch-glass, all ready, as the opera- 

 tion must be carried through quickly. I select a bit of the sea- 

 weed, just large enough for the mount ; put a drop or more of 

 the chloroform in the watch-glass, and immerse the bit. The 

 chloroform seems to be as efficient as water in restoring the dried 

 alga to its natural shape. As the chloroform evaporates rapidly 



