At the present time the instruments of Thoma-Zeiss or others similarly constructed are generally used; and we may assume that the principle on which they depend and the methods of their use are known. A number of fluids are used to dilute the blood, which on the whole fulfil the requirements of preserving the form and colour of the red corpuscles, of preventing their fusing together, and of allowing them to settle rapidly. Of the better known solutions we will here mention =Pacini's= and =Hayem's= fluids. Pacini's solution. Hydrarg. bichlor. 2.0 Natr. chlor. 4.0 Glycerin 26.0 Aquae destillat. 226.0 Hayem's solution. Hydrarg. bichlor. 0.5 Natr. sulph. 5.0 Natr. chlor. 1.0 Aquae destillat. 200.0 For counting the white blood corpuscles the same instrument is generally used, but the blood is diluted 10 times instead of 100 times. It is advantageous to use a diluting fluid which destroys the red blood corpuscles, but which brings out the nuclei of the white corpuscles, so that the latter are more easily recognised. For this purpose the solution recommended by Thoma is the best--namely a half per cent. solution of acetic acid, to which a trace of methyl violet has been added[1]. The results of these methods of enumeration are sufficiently exact, as they have, according to the frequently confirmed observations of R. Thoma and I. F. Lyon, only a small error. In a count of 200 cells it is five per cent., of 1250 two per cent., of 5000 one, and of 20,000 one-half per cent. There are certain factors in the practical application of these methods, which in other directions influence the result unfavourably. It has been found by Cohnstein and Zuntz and others that the blood in the large vessels has a constant composition, but that in the small vessels and capillaries the formed elements may vary considerably in number, though the blood is in other respects normal. Thus, for example, in a one-sided paralytic, the capillary blood is different on the two sides; and congestion, cold, and so forth raise the number of red blood corpuscles. Hence, for purposes of enumeration, the rule is to take blood only from those parts of the body which are free from accidental variation; to avoid all influences such as energetic rubbing or scrubbing, etc., which alter the circulation in the capillaries; to undertake the examination at such times when the number of red blood corpuscles is not influenced by the taking of food or medicine.