The Gardner Sunitarium

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corpuscle (Fig. 14, W) would have a diameter therefore of Totooth 25th of an inch. It is obvious, however, that with such magnification a red blood-corpuscle, if its diameter beth of an inch, will be covered by less than four such spaces, and by more than three, since th of an inch is less than 1th and more than 18th of an inch. As the red corpuscles, in order to be measured, must lie within the space between two lines the value of which is known, and further, as the edges of the corpuscle must be exactly in contact with the two lines circumscribing it, an object-glass and length of tube must be

FIG. 14.

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W

Lines of Eye-Piece Micrometer, projected upon Stage Micrometer, as seen with different magnification.

used so that exactly 32 lines of the eye-piece micrometer can be counted within a space of 10th of an inch, as each space will then be equal to th of an inch, and will exactly cover the red corpuscles (Fig. 14, R).

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00

Such a simple micrometric arrangement as that just described (Fig. 14) will suffice for determining whether bodies supposed to be red corpuscles have an average diameter of 20th of an inch, and, approximately at least, the diameter of larger or smaller bodies. It is obvious, however, that if the body to be measured was theth of an inch in diameter, in order to measure it accurately the magnification (Fig. 14) would have to be so altered that the space of the 5th of an inch would contain exactly thirty lincs instead of thirty-two. To

avoid the inconvenience of altering the magnification in each case which alteration would necessitate altering the length of the tube or changing the objective, or both

B, C, lines upon stage micrometer; 4, fixed line in ocular; a, b, movable lines in ocular.

microscopists make use of a form of micrometer essentially the same as that used by astronomers in measuring the apparent diameter of the heavenly bodies. This consists of an ocular (Fig. 15, 1) through which may be seen

projected upon the stage micrometer a fixed line A, and two movable lines a, b. The latter can be moved across the field of the microscope by means of a wheel (not represented in the figure), the distance traversed by either of the threads from the fixed line being determined by the number of divisions through which the wheel is rotated (Fig. 15, 3). As an illustration of the manner in which this is accomplished, let us suppose that the thread b is made to coincide with the fixed line A of the ocular, and the latter is made to coincide at the same time with the line B of the stage micrometer (Fig. 15, 2), and that the wheel is turned through one hundred divisions; that is, makes one complete rotation. It will be observed that the line b will traverse the space between the lines B and C of the stage micrometer, stopping at and coinciding with the line C. By the turning of the wheel through one hundred divisions the line b has, therefore, been made to traverse the space of the 6th of an inch, the space between B and C of the stage micrometer having been so graduated; consequently, if the wheel be turned through ten divisions, the line b will be made to traverse the Toth of an inch, and so on proportionally. The relation between the space traversed by the line b of the ocular and the number of divisions through which the wheel is turned having been experimentally determined by means of the stage micrometer, the latter is removed, and the object-glass holding the object to be measured is substituted in the field of the microscope. To determine the size of the latter, it is only necessary then to turn the wheel until the body to be measured (Fig. 15, 3) is exactly circumscribed by the fixed line A and the movable line b of the ocular, and to read off the number of divisions through which the wheel has been turned to accomplish

It is evident, however, that if the wheel must be rotated through twenty divisions and a fraction of a division in order to bring the body to be measured between the lines A and b, that fraction might be such that even with a vernier attached to the wheel it would be impossible to get an exact reading, and consequently the measurement would only be approximate.

It is a matter, however, of the greatest difficulty with high powers to adjust accurately the divisions of the eye-piece micrometer, whatever form of instrument may be used, to either those of the stage micrometer or to the margins of the objects to be measured, even with all the ingenious accessory contrivances that have been devised to facilitate the operation. Indeed, it must be admitted that the measurement of so small a body as the red corpuscle, even when made by a most skilful microscopist and with the best of modern instruments, from the very nature of the case can never be anything but an approximate one. If the measurement of the red bloodcorpuscle from freshly-drawn blood and under the most favorable circumstances is at best only approximative, how much more so must such measurement be in the case of a blood-stain where the size of the blood-corpuscle will depend upon the relative amount of the fluid absorbed that was used in its preparation for microscopical examination? Indeed, one of the greatest difficulties experienced in restoring the form of the blood-corpuscles obtained from a blood-stain is to prevent them becoming distorted, swollen, or even bursting from excessive absorption of the fluid used in their preparation. The size of a corpuscle,

as obtained from a blood-stain, can only be regarded then as representing approximately the size of the corpuscles of such blood. Further, it must be borne in mind, in this connection, that while about ninety out of every hundred red corpuscles, whether the blood be that of man or other mammals, have the same diameter, the latter depending upon the species; of the remaining ten corpuscles, some are larger, some smaller than the average corpuscle. That being the case, if it just so happened that only the exceptionally small corpuscles were present, the blood, though human, might be erroneously regarded, on account of the small size of the corpuscles, as that of a dog, for example, in which the corpuscles are smaller than those of man. On the other hand, if the blood examined was that of a dog, but only the exceptionally large corpuscles were obtained, such blood, on account of the large size of its corpuscles, might improperly be considered as human.

It must be admitted, therefore, that while the red bloodcorpuscles of the mammalia can be shown by measurement to differ in size (see table), the blood examined being freshly drawn in each instance, red blood-corpuscles as obtained from blood-stains cannot be positively identified by such a method as human red blood-corpuscles. Any evidence offered as positive proof based upon micrometric methods that blood is human, as distinguished from other mammalian blood, must be regarded as only circumstantial at best, for the following three reasons mentioned above: 1. The micrometric method is approximative. 2. The size of the corpuscle restored is variable, depending upon the amount of fluid absorbed. 3. The size of the corpuscles varies, even in the blood of the same mammal.