MICROSCOPICAL TECHNIQUE. 425 



ration is effected in six hours, and, at the end of twenty-four 

 hours, three Uquid layers have formed. The lowest is clear, and of 

 a yellow colour, the middle one somewhat turbid, whilst the upper 

 one, which is clear and colourless, contains the whole of the fat. 

 An aliquot weight of the latter (petroleum solution) is then evapo- 

 rated in a flat dish at 50°, and the residue dried at 100*^, Instead 

 of weighing the fat solution, 25 c.c. of the milk is shaken with 

 3 c.c. of sodium hydroxide solution, and subsequently with three 

 portions of light petroleum (25 c.c. each). 50 c.c. of the fat solu- 

 tion is then evaporated, and the fat weighed and calculated to 

 100 c.c. of milk. 



The Brownian Movement.^— Mr. A. B. Chapin gives the fol- 

 lowing method for making a slide showing the Brownian Move- 

 ment : — Take two perfectly clean one-ounce bottles ; into No. i 

 put half an ounce of distilled water, ten drops of the tincture tolu, 

 four drops of pure carbolic acid, and filter through absorbent cotton 

 into No. 2. Make a very thin glass cell, and cement it to the slide 

 with hard marine glue. Now put the slide on turn-table, and spin 

 a very shallow ring of good gold size on the outer edge of the cell 

 and allow it to dry until just ' tacky ' enough to adhere well to the 

 cover-glass. Put into this cell enough of the solution to fill, care- 

 fully adjust the cover-glass, and clamp down very lightly. Absorb 

 any surplus that may have run out with blotting-paper, set aside 

 for a few days, then ring, and you have a permanent mount. 



Air-Bubbles and Oil-Globules. t— It is of importance to be able 

 to identify and distinguish between air-bubbles and oil-globules in 

 preparations under microscopical investigation. The appearances 

 of both vary considerably according to the portion of them that 

 happens to be in focus. Dallinger in The Microscope and its 

 Revelations represents and describes these different aspects, as 

 presented when light is transmitted from a concave mirror exactly 

 centred (axial illumination), and a diaphragm of about two-thirds 

 of a mm. is placed at a distance of 5 mm. beneath the stage. 

 This will represent the smallest opening if a wheel diaphragm be 

 used, whilst an iris diaphragm should be almost completely closed. 

 Air-bubbles in water and Canada balsam respectively may be 



* The Observer, iv. (1893), P- I99- i' Pharmaceutical Journal. 



