SOME DIATOMACE^ FROM CORICA BAY, MELBOURNE. 297 



In studying minute diatomic structure witli oil-immcrsion 

 objectives and condensers, especially with the latter, type or group 

 slides containing a large number of varied forms are a great 

 convenience, as valve after valve can be critically examined 

 without changing the slip — a somewhat tiresome operation when 

 both its upper and under surfaces are in immersion contact with 

 objective and condenser. The beautifully arranged and compara- 

 tively cheap grouped " circle " slides, containing from fifty to two 

 hundred selected forms from numerous localities, are admirably 

 adapted for this purpose. Every rose has its thorn, however, and 

 the thorn in this case proves to be the fact that a large percentage 

 of the valves arranged on such slides are so mounted as to present 

 their concave inner surfaces to the observer, whereas, at any rate 

 where only one example of each form is given, it is very desirable 

 that the valve should be mounted with its outer convex surface 

 uppermost. The utility of a beautiful type slide of four hundred 

 forms, which I possess, is greatly diminished owing to this cause, 

 many most interesting specimens on it being thus mounted inside 

 out. The marvellous skill evinced in the arrangement of these 

 preparations leads one to hope that mounters may be able to 

 remedy this defect, especially in the more elaborate and expensive 

 type slides. 



When examining very minute objects with objectives of the 

 highest power, employing large axial cones, I have found slightly 

 averted vision to be of great assistance in steadily holding faint 

 and difficult details. The utility of averted vision is of course 

 w^ell known in telescopic observations, but I have not heard of it 

 hitherto being employed in microscopical work. In my own case 

 I find that faint diatomic structure which can be just certainly 

 held with averted vision becomes absolutely invisible when viewed 

 directly. 



I have been lately again working at the secondary structure 

 on the i\^. 2^TfPtexta and X. lyra, and have succeeded in holding 

 it as distinct dots in both cases. My impression is, however, 

 that while the secondaries all over the upper surface of the 

 ^V. iwietexta represent real structure, those of the X. hjra banda 

 may be '' false ghosts " formed by the adjacent primaries, as they 

 are arranged across the bands in close rows parallel to the coarse 

 structure ; but, on the other hand, different specimens of the 

 X. lyra, all possessing well-marked primaries, do not exhibit the 



