86 NOTES AND MEMORANDA. 



piece well. It is to be supposed that these are at least not ' short 

 tubes,' and are good evidence that the definition is obtained by quality 

 of the objective." 



How to pick out Diatoms in Mounting.— Dr. C. Johnston in a 

 paper on this subject (' Lens,' November), says that nothing is easier 

 than to seize particular diatoms and transfer them to a bottle for 

 future use, or to a slide, provided the field from which we select be 

 rich and clean. Difficulty, however, occurs when forms in any gather- 

 ing are few and far between. Let such prepared material be spread 

 upon a large slide, covering a space of one inch by two, and let it be 

 filliped as it is set away to dry spontaneously. With a f-inch ob- 

 jective, search the white field for any diatoms whatever, and, upon 

 finding, encircle each one with a line, made with a point of a match 

 sharpened and moistened, adding near the circle a dot, or cross, or 

 other sign, always appropriated to the same diatom, and of which a 

 tallying record is kept on paper. At leisure one may, without trouble, 

 single out any desired object, pick it off with a fine dampened point 

 of cane (reed), not including the silicious cuticle, and deposit it, free 

 from injury, in a small drop of distilled water placed in the centre of 

 the slide. 



The Use of Osmic Acid. — Osmic acid when obtained pure is a 

 yellow crystalline solid, which is volatile at ordinary temperatures, 

 has a peculiar stinking odour, and rapidly excites a severe and per- 

 sistent catarrh in those exposed to its slightest influence. It is better, 

 therefore, says Col. Woodward (' Lens,' November), for the miero- 

 scopist to procure it from the dealer already made into a one-per-cent. 

 solution, which he can dilute at pleasure for use. Even dilute solu- 

 tions, however, have an unpleasant smell, and excite catarrh unless 

 great care is taken to avoid exposure. A box outside the window of 

 the working-room is the proper place to set capsules containing por- 

 tions of tissue during the action of the reagent, which should always 

 be handled near an open window. The preparation to be acted upon 

 should be as fresh as possible, and laid in a small quantity of the 

 solution for several hours, when it may be washed with water, and is 

 ready for examination in water or glycerine. According to the inten- 

 sity of action desired, the solution may vary in strength from one-half 

 to one-tenth of one per cent. It will be found to have a particularly 

 energetic action on the medullary sheath of the nerves, which ac- 

 quires a peculiar purplish-brown colour passing into black if the 

 action is very intense, while the surrounding tissues are but little 

 stained or remain quite uncoloured. Fatty matters of all kinds are 

 also blackened by the acid, the use of which, therefore, is undesirable 

 if the part to be investigated contains much adipose tissue. 



Frustulia Saxonica as a Definition Test. — Dr. Woodward does 

 not appear to set so high a value on this test as the Germans do. He 

 states (' Lens,' November), that having been lately presented with an 

 opportunity of examining specimens of the diatom, he found no diffi- 

 culty in seeing and counting the transverse striae, both with mono- 

 chromatic sunlight and with the light of a small coal-oil lamp. The 



