The =red cells= may be measured by the ocular micrometer. Those of man are somewhat larger than those of other mammals, but the difference in size is so slight that an absolute determination of the source of the blood is not possible from the consideration of size alone. Human corpuscles measure 0.0077 mm. in diameter; the nearest in size are the corpuscles of the dog, rabbit, hog, cow, horse, cat and sheep, in the order given, those of the sheep being smaller. The measurements of a large number of corpuscles must be taken if the question of size is to be considered. When red cells cannot be found the stains must be tested for the presence of blood =pigment= (_Teichmann’s hæmin crystals_). A small piece of the scraping or stained fabric is placed upon a slide in glacial acetic acid; the fluid is expressed; and a small crystal of sodium chloride or sodium iodide is added to the fluid. Cover with cover-glass and heat gently until bubbles are given off, and continue carefully until the acetic acid has evaporated. The slower the evaporation the larger the crystals will be. Examine with a ⅙ inch or No. 7 objective. The hæmin crystals are brown or claret-colored, of the shape of rhombic plates single or superimposed. They are insoluble in water, alcohol and ether; dissolve slowly in ammonia, dilute sulphuric and nitric acids, and easily in caustic potash. The form of the crystal and the color determines the diagnosis of the presence of blood, but does not distinguish between human and animal blood. Old stains should be treated with acetic acid 12-24 hours before attempting to obtain the crystals. Contamination of the fabric with fat interferes with the reaction; the fat should be removed first with ether. If the suspected stain occurs on iron, steel, sand, clay or coal, the crystals usually cannot be obtained. Decomposition may also interfere with the reaction. In such cases a solution of the stain in a saturated solution of borax should be made and submitted to spectroscopic examination. The presence of blood-pigment on any substance can be demonstrated by means of the =spectroscope= or =microspectroscope=. Within recent years the =biologic= test for blood and albumins has been turned to medicolegal uses, in the differential diagnosis of human and animal blood or albumins. This test rests upon the principle that in the serum of an animal treated with human blood or albumin there are developed antibodies having a strong specific hæmolytic or precipitating action upon human blood or albumin.