212 A METHOD OF RESOLVING DRY DIATOMACEOUS TESTS. 



are easily lined or checked, according to whether their mid-ribs are 

 lying in the direction of the image of the flame, or at an angle of 

 about 45° to the latter. Even on damp slides of Stauroneis 

 spicula (of which there seem to exist more than of good ones), the 

 cross-lines on this diatom may be readily and distinctly seen. 



Surirella gemma assumes an olive-green colour on that part which 

 is in close contact with the covering glass, and shows rows of 

 exquisite, small, and regular hemispherical beads, but by no means 

 hexagonal markings. 



Navicula cuspidata assumes a beautiful iridescence, and shows its 

 elongated beads to perfection, and is altogether a lovely object 



Pleurosigma angulation shows its markings as regular, small, and 

 widely-separated beads. Take any diatom you like, and the result 

 on that part with which it adheres to the cover is equally satisfactory 

 and often surprising. It is easily seen whether an object adheres to 

 the cover or not, as in the latter case its outlines appear ill-defined, 

 and the whole as in a haze. 



1 sum up the advantages of the mode of illumination here described 

 under the following heads : — 



1. The correctness with which objects and their details are seen. 



2. The ease with which the most difficult resolutions are obtained, 

 as the veriest tyro in microscopical illumination may, with a suitable 

 objective, resolve the most difficult diatomaceous tests known, in a 

 few minutes time. 



3. The simplicity and cheapness of the apparatus employed, the 

 vertical illuminator being obtainable at so low a cost as about 

 twelve shillings. 



4. The adaptability of the illlnminating apparatus employed to the 

 simplest stands. 



5. The value of this method as a means of expeditiously testing 

 objectives with regard to the utmost limits of their resolving power, 

 and under the most favourable illumination, because the illuminating 

 ray impinges on the object at just that greatest angle at which the 

 objective is capable of receiving, or of transmitting the same. 



