material before us we may draw the conclusion, that after long residence in elevated districts the number of red blood corpuscles is absolutely raised. The therapeutic importance of this influence is obvious.

Besides high altitudes, the influence of the tropics on the composition of the blood and especially on the number of corpuscles has also been tested. Eykmann as well as Glogner found no deviation from the normal, although the almost constant pallor of the European in the tropics points in that direction. Here also, changes in the distribution occurring without qualitative changes of the blood seem chiefly concerned.

The same reliance cannot be placed on inferences based on the results of the Thoma-Zeiss and similar counting methods for anæmic as for normal blood, in which generally speaking all the red cells are of the same size and contain the same amount of hæmoglobin. In the former the red corpuscles, as we shall shew later, differ considerably one from another. On the one hand forms poor in hæmoglobin, on the other very small forms occur, which by the wet method of counting cannot even be seen.

Apart even from these extreme forms, 1,000 red blood corpuscles of anæmic blood are not physiologically equivalent to the same number of normal blood corpuscles. Hence the necessity of closely correlating the result of the count of red blood corpuscles with the hæmoglobinometric and histological values. The first figure only, given apart from the latter, is often misleading, especially in pathological cases.

It is therefore occasionally desirable to supplement the data of the count by the estimation of the size of the red blood corpuscles individually. This is effected by direct measurement with the ocular micrometer; and can be performed on wet (see below), as well as on dry preparations, though the latter in general are to be preferred on account of their far greater convenience.

Nevertheless the carrying out of this method requires particular care. One can easily see that in normal blood the red corpuscles appear smaller in the thicker than they do in the thinner layers of the dry preparation. We may explain this difference as follows. In the thick layers the red discs float in plasma before drying, whilst in the thinner parts they are fastened to the glass by a capillary layer. Desiccation occurs here nearly instantaneously, and starts from the periphery of the disc; so that an alteration in the shape or size is impossible. On the contrary the process of drying in the thicker portions proceeds more slowly, and is therefore accompanied by a shrinking of the discs.

Even in healthy persons small differences in the individual discs are shewn by this method. The physiological average of the diameter of the greater surface is, according to Laache, Hayem, Schumann and others, 8.5 µ for men and women (max. 9.0 µ. min. 6.5 µ.) In anæmic blood the differences between the individual elements become greater, so that to obtain the average value, the maxima, minima, and mean of a large number of cells, chosen at random, are ascertained. But with a high degree of inequality of the discs this microscopical measurement loses all scientific value.

However valuable the knowledge of the absolute number may be for a judgment on the course of the illness, it gives us no information about the amount of hæmoglobin in the blood, which is the decisive measure of the degree of the anæmia. A number of clinical methods are in use for this estimation; first direct, such as the colorimetric estimation of the amount of hæmoglobin, secondly indirect, such as the determination of the specific gravity or of the volume of the red corpuscles, and perhaps also the estimation of the dry substance of the total blood.

Among the direct methods for hæmoglobin estimation, which aim at the measurement of the depth of colour of the blood, we wish first to mention one, which though it lays no claim to great clinical accuracy has often done us good service as a rapid indicator at the bedside. A little blood is caught on a piece of linen or filter-paper, and allowed to distribute itself in a thin layer. In this manner one can recognise the difference between the colour of anæmic and of healthy blood more clearly than in the drop as it comes from the finger prick. After a few trials one can in this way draw conclusions as to the degree of the existing anæmia. Could this simple method which is so convenient, which can be carried out at the time of consultation, come more into vogue, it alone would contribute to the decline of the favourite stop-gap diagnosis, 'anæmia.' For neurasthenic patients also, who so often fancy themselves anæmic and in addition look so, a demonstratio ad oculos such as this is often sufficient to persuade them of the contrary.

Of the instruments for measuring the depth of colour of the blood, the double pipette of Hoppe-Seyler is quite the most delicate. A solution of carbonic oxide hæmoglobin, accurately titrated, serves as the standard of comparison. The reliable preparation and conservation of the normal solution is however attended with such difficulties, that this method is not clinically available. In the last few years, Langemeister, a pupil of Kühne's, has invented a method for colorimetric purposes, also applicable to hæmoglobin estimations. The instrument depends on the principle, that from the thickness of the layer in which the solution to be tested has the same colour intensity as a normal solution, the amount of colour can be calculated. As a normal solution Langemeister uses a glycerine solution of methæmoglobin prepared from pig's blood. To our knowledge this method has not yet been applied clinically. Its introduction would be valuable, for in practice we must at present be content with methods that are less exact, in which coloured glass or a stable coloured solution serves as a measure for the depth of colour of the blood. There are a number of instruments of this kind, of which the "hæmometer" of Fleischl, and amongst others, the "hæmoglobinometer" of Gowers, distinguished by its low price, are specially used for clinical purposes. Both instruments give the percentage of the hæmoglobin of normal blood which the blood examined contains, and are sufficiently exact in their results for practical purposes and for relative values; although errors up to 10% and over occur with unpractised observers. (Cp. K. H. Mayer.) Quite recently Biernacki has raised the objection to the colorimetric methods of the quantitative estimation of hæmoglobin, that the depth of colour of the blood is dependent not only on the quantity of hæmoglobin but also on the colour of the plasma, and the greater or less amount of proteid in the blood. These errors are quite inconsiderable for the above-mentioned instruments, since here the blood is so highly diluted with water that the possible original differences are thereby reduced to zero.

Among the methods for indirect hæmoglobin estimation, that of calculation from the amount of iron in the blood appears to be quite exact, since hæmoglobin possesses a constant quantity of iron of 0.42 per cent. This calculation may be allowed in all cases for normal blood, for here there is a really exact proportion between the amounts of hæmoglobin and of iron. Recently A. Jolles has described an apparatus for quantitative estimation of the iron of the blood, called a "ferrometer;" which renders possible an accurate valuation of the iron in small amounts of blood. However for pathological cases this method of hæmoglobin estimation from the iron present is not to be recommended. For if one tests the blood