Remarks on the Measurement of Minute Particles, especially those of the Blood and of Pus. From Dr. Young's Medical Literature, 8vo. Lond. 1813, p. 545.

I. On the Form and Magnitude of the Particles of the Blood. THE form and magnitude of the coloured particles of the blood is a subject not only interesting and important in itself, but is also capable of assisting, by means of comparative observations, in the determination of the magnitude of the capillary arteries, and the investigation of the resistance which they exhibit; it may also be of advantage to obtain some tests capable of ascertaining, whether these particles undergo any change in diseases of various kinds, and what is their relation to the globules of pus, and of other animal fluids: hitherto the measures of the particles of blood, which have been considered by various authors as the most accurate, have differed no less than in the ratio of 2 to 5; and there is an equal degree of uncertainty respecting their form, some admitting the truth of Mr. Hewson's opinions, and a greater number rejecting them without any satisfactory evidence. In such examinations, it is only necessary to employ a full and unlimited light, in order to obtain a very distinct outline of what appears manifestly to be a very simple substance, and we thus seem to have the clear evidence of the senses against Mr. Hewson: but we must remember, that where the substances to be examined are perfectly transparent, it is only in a confined and diversified light that we can gain a cor

rect idea of their structure. The eye is best prepared for the investigation, by beginning with the blood of a skate, of which the particles are so conspicuous, and of so unequivocal a form, as to set aside at once the idea of a simple homogeneous substance. They are oval and depressed, like an almond, but less pointed, and a little flatter; each of them contains a round nucleus, which is wholly independent in its appearance of the figure of the whole disc, being sometimes a little irregular in its form; seldom deviating from its central situation, but often remaining distinctly visible while the oval part is scarcely perceptible; and as the portion of blood dries away, becoming evidently prominent above the thinner portion. This nucleus is about the size of a whole particle of the human blood, the whole oval being about twice as wide, and not quite three times as long; the nucleus is very transparent, and forms a distinct image of any large object which intercepts a part of the light by which it is seen, but exhibits no inequalities of light and shade, that could lead to any mistake respecting its form. But if we place some particles of human blood under similar circumstances, near the confine of light and shade, although they are little, if at all, less transparent, we immediately see an annular shade on the disc, which is most marked on the side of the centre on which the marginal part appears the brightest, and consequently indicates a depression in the centre, which Delatorre mistook for a perforation. It is most observable when the drop is drying away, so that the particles rest on the glass: and when a smaller particle is viewed, it has merely a dark central spot, without any lighter central space. Nor have the particles ever appeared to me "as flat as a guinea," although their axis is sometimes not more than one-third or one-fourth of their greatest diameter; if they were much thinner than this, their diameter would be more diminished than it is when they become spherical, by the effect of an aqueous fluid: while this form corresponds to a diminution to about of the original diameter. They may indeed possibly absorb a part of the surrounding moisture in the change: but they do not seem to have their dimensions much affected by the fluid in which they are suspended, since they may easily be spread thin on glass, and dried, without much change of their magnitude, at least in the direction of the surface to which they adhere: and they remain distinct as long as the access of moist air is completely excluded.. When they have been kept for some time in water, and a little solution of salt is added, their form and structure, as Mr. Hewson has observed, are more easily examined, and appear to resemble those of a soft substance with a denser nucleus, not altogether unlike the crystalline lens together with the vitreous humour, as seen from behind: but with respect to a central

particle detached within a vesicle, "like a pea in a bladder," I cannot doubt that Mr. Hewson was completely mistaken. I have never observed a prominence in the outline of the particles of the human blood and on the other hand I am not perfectly confident that the apparent depression, which is exhibited in some lights, may not depend on some internal variation of the refractive density of the particle. It has commonly been asserted, that these coloured particles are readily soluble in water; but this opinion appears to be completely erroneous, and to depend partly on their passing readily through filtering paper, a circumstance indeed already observed by Berzelius, (Djurk. ii. p. 3,) and partly on the extraction of a great part of their colouring matter, together with which they lose much of their specific gravity, so that instead of subsiding, they are generally suspended in the fluid; their presence may, however, still be detected by a careful examination, and they seem in this state to have recovered in some measure their original form, which they had lost when first immersed in the water. When the water is sufficiently diluted, about three-fourths as much rectified spirits may be added to it without destroying the appearance; but after a few months it becomes indistinct, although neither in this case nor in that of complete putrefaction do the globules appear to become constituent parts of a homogeneous fluid. The existence of solid particles, in fluids which at first sight appear transparent, is the most easily detected, by looking through them at a small luminous object, either directly or by reflection, as, for example, at the image of a candle seen at the edge of the portion of the fluid, held in a tea-spoon; in this case, wherever there are small particles in suspension, for instance, in milk diluted with water, they will produce a minutely tremulous or sparkling appearance, which is rendered still more distinct by the assistance of a lens, and which depends on the diversified interception of the light, while the particles are carried over each other by the internal motion of the fluid. This test is applicable to all cases of minute particles held in suspension; where however the greater number of the particles are nearly equal in dimensions, the luminous object viewed through them exhibits a much more striking appearance, for it is surrounded by rings of colours, somewhat resembling those of the rainbow, but differently arranged, and often beautifully brilliant. The blood, a little diluted, always exhibits them in great perfection, and they afford a very accurate criterion for the distinction between pus and mucus: mucus, containing no globules, affords no colours, while those which are exhibited by pus exactly resemble the appearance produced by the blood, the rings being usually of the same dimensions : whence it follows that the globules are also of the same size, for the dimensions of the rings vary with those of the particles

which produce them: and there can be little doubt, from this circumstance, that the globules found in pus are the identical globules of the blood, although probably somewhat altered in the process of suppuration. A minute quantity of the fluid to be examined in this manner may be put between two small pieces of plate glass, and if we hold the glass close to the eye, and look through it at a distant candle, with a dark object behind it, the appearance, if the globules are present, will be so conspicuous as to leave no doubt respecting their existence.

II. Description of an Eriometer.

The rings of colours, which are here employed to discover the existence of a number of equal particles, may also be employed for measuring the comparative and the real dimensions of these particles, or of any pulverised or fibrous substances, which are sufficiently uniform in their diameters. Immediately about the luminous object, we see a light area, terminating in a reddish dark margin, then a ring of bluish green, and without it a ring of red: and the alternations of green and red are often repeated several times, where the particles or fibres are sufficiently uniform. I observed some years ago that these rings were the larger as the particles or fibres affording them were smaller, but that they were always of the same magnitude for the same particles. It is therefore only necessary to measure the angular magnitude of these rings, or of any one of them, in order to identify the size of the particles which afford them; and having once established a scale, from an examination of a sufficient number of substances of known dimensions, we may thus determine the actual magnitude of any other substances which exhibit the colours. The limit between the first green ring, and the red which surrounds it, affords the best standard of comparison, and its angular distance may be identified, by projecting the rings on a dark surface, pierced with a circle of very minute holes, which is made to coincide with the limit, by properly adjusting the distance of the dark substance, and then this distance, measured in semidiameters of the circle of points, gives the corresponding number of the comparative scale. Such an instrument I have called an Eriometer, from its utility in measuring the fibres of wool, and I have given directions for making it, to Mr. Fidler, in Foley-street. The luminous point is afforded by a perforation of a brass plate, which is surrounded by the circle of minute holes ; the substance to be examined is fixed on some wires, which are carried by a slider, the plate being held before an Argand lamp, or before two or three candles placed in a line; the slider is drawn out to such a distance as to exhibit the required coincidence, and the index then shows the number representing the magnitude of the substance examined. The

instrument may be rendered more portable, though somewhat less accurate, by merely making the perforations in a blackened card, furnished with a graduated piece of tape. An eye not short-sighted will generally require the assistance of a lens, when the instrument is made of the most convenient dimensions, which I have found to be such as to have two circles of points, one at and the other of an inch in semidiameter, with their corresponding scales. The central perforations are about and of an inch in diameter; the points 8 or 10 only in each circle, and as minute as possible. The light of the sun might also be employed, by fixing the circle of points at the end of the tube of a telescope: but it rather adds glare than distinctness to the colours: nor have I been able to gain any thing by looking through coloured glasses, or by using lights of different qualities. Where the object consists of fibres which can be arranged in parallel directions, a fine slit in the plate or card affords brighter colours than a simple perforation, and the points must in this case be arranged in lines parallel to the slit; but if care is not taken to stretch the fibres sufficiently, the employment of the slit in this manner will make them appear coarser than they really are. The colours will still appear, even if there be a considerable difference in the dimensions of the fibres or particles, but they will be so much the less distinct as the difference is greater. In this case the measure indicated will be intermediate between the extreme dimensions; although most commonly it will be seen somewhat below the true mean, the colours exhibited by the finer fibres prevailing in some degree over the rest. The latitude, however, which the eriometer affords in the regularity of the substances measured by it, and its collecting into one result the effect of many thousands of particles, or of an endless variety of small differences in the diameters of fibres, give it an unquestionable preference over every kind of micrometer which measures a single interval only at once, with respect to all applications to agriculture or manufactures; for in reality there is not a single fibre of wool among the millions which constitute a fleece, that preserves an uniform diameter throughout its length, and the difference is still greater between the fibres which grow on different parts of the animal;" so that to take a single measurement, or even any practical number of measurements, by the most accurate micrometer, in the usual acceptation of the term, for a criterion of the quality of a fleece, can tend only to the propagation of error or conjecture in the semblance of the minutest accuracy. Even with the eriometer, the difficulty of obtaining a fair average of the quality of a sample of wool is extremely great; it is absolutely necessary to preserve the fibres as much as possible in their natural relative situation, and to examine them near the middle of their