is best, however, to examine several drops; and, when the sediment is abundant, drops from the upper and lower portions should be examined separately. In examining urinary sediments microscopically no fault is so common, nor so fatal to good results, as improper illumination (see Figs. 2 and 3), and none is so easily corrected. The light must be central and very subdued. The two-thirds objective should be used as a finder, while the one-sixth is reserved for examining details. It is well to emphasize that _the most common errors which result in failure to find important structures, when present, are lack of care in transferring the sediment to the slide, too strong illumination, and too great magnification_. {103} In order to distinguish between similar structures it is often necessary to watch the effect upon them of certain reagents. This is especially true of the various unorganized sediments. They very frequently cannot be identified from their form alone. With the structures still in focus, a drop of the reagent may be placed at one edge of the cover-glass and drawn underneath it by the suction of a piece of blotting-paper touched to the opposite edge; or a small drop of the reagent and of the urine may be placed close together upon a slide and a cover gently lowered over them. As the two fluids mingle, the effect upon various structures may be seen. Urinary sediments may be studied under three heads: A. Unorganized sediments. B. Organized sediments. C. Extraneous structures. A. UNORGANIZED SEDIMENTS In general these have little diagnostic or prognostic significance. Most of them are substances normally present which have been precipitated from solution either because present in excessive amounts or, more frequently, because of some alteration in the urine (as in reaction, concentration, etc.) which may be purely physiologic, depending upon changes in diet or habits. Various substances are always precipitated during decomposition, which may take place either within or without the body. Unorganized sediments may be classified according to the reaction of the urine in which they are most likely to be found: In acid urine: uric acid, amorphous urates, sodium urate, calcium oxalate, leucin and tyrosin, cystin, and fat-globules. Uric acid, the urates, and calcium oxalate are {104} the common deposits of acid urines; the others are less frequent, and depend less upon the reaction of the urine. In alkaline urine: phosphates, calcium carbonate, and ammonium urate. Other crystalline sediments (Fig. 32) which are rare and require no further mention are: calcium sulphate, cholesterin, hippuric acid, hematoidin, fatty acids, and indigo. [Illustration: FIG. 32.--Unusual urinary crystals (drawn from various authors): 1, Calcium sulphate (colorless); 2, cholesterin (colorless); 3, hippuric acid (colorless); 4, hematoidin (brown); 5, fatty acids (colorless); 6, indigo (blue); 7, sodium urate (yellowish).]