is that of a thin brass slide holding the ruled glass, which is inserted through a slit in the side of the eye-piece and has at one end a screw by which the micrometer can be given a slight lateral motion. The object to be measured is observed through the glass micrometer; one side of the object to be measured is brought exactly up to one of the lines, and the number of spaces which the object covers is carefully counted.

The value of the spaces of the eye-piece micrometer is merely relative, and dependent upon different objectives and varying adjustments of the instrument. The value of the spaces is determined by a stage micrometer. This consists of a piece of ruled glass with spaces of 1-1000 of an inch. These spaces, when magnified and seen through the eye-piece micrometer, are covered by a certain number of lines in the latter. With a given objective, a 1-1000 space, for example, may be magnified until it covers five lines in the eye-piece; one of the latter spaces, therefore, measures 1-5000 of an inch. In most cases the eyepiece micrometer will not cover any certain number of full spaces, but there will be so many spaces and a fractional part of another space.

In such cases the draw-tube of the instrument must be extended until the increased amplification makes the space of the stage micrometer

B

Fig. 18. Eye-piece
Micrometer.

equal to a number of full spaces in the eye-piece. For convenience of calculation the number of these should, if possible, be some multiple of five or ten. If, under a high power (2500 diameters), forty spaces in the eye-piece are required to cover one space (1-1000 inch) of the stage micrometer, one space of the former will indicate 1-40,000 of an inch. Fig. 18, A, represents a portion of an eye-piece micrometer under a micrometry of 40,000, and the concentric circles the outlines of blood-corpuscles of man, the dog, rabbit, ox, and sheep. The largest corpuscle covers 12 spaces; its measurement is therefore 12.33-40,000 of an inch. Reducing this fraction to its simplest form, we obtain 1-3244. In the same manner we find that the corpuscle of the sheep covers 7 spaces, and therefore measures 7.75-40,000, which equals 1-5161.

123

We cannot enter further into the minute details of micrometry, but must not omit a single most important suggestion. No perfect stage micrometer was ever produced, and the observer must carefully compare and calculate the true value of each space which forms the basis of his measurements. An otherwise valuable piece of testimony may be completely shattered by a single question on cross-examination: "If you have not verified your micrometer, how do you know that your measure ments are correct?"

THE DISTINCTION BETWEEN HUMAN AND OTHER BLOOD.

It is only by the microscope that any satisfactory distinction can be made between human and other blood. Various methods have been suggested depending upon other principles, but they have been found to be unreliable or impracticable in their application to dried stains. Barruel's process depends upon the production of an odor characteristic

of the particular animal when fresh blood is mixed with sulphuric acid. (Annales de Hygiene, 1829.) Taddei proposed to distinguish human blood from that of animals by the degree of fluidity produced in a compound of blood and carbonate of copper when treated with sulphuric acid. (For an account of Taddei's process, called hæmatolloscopy, and also Barruel's method, see Fleming's article on blood-stains in Am. Jour. Med. Sciences, vol. xxxv., p. 98, 1859.) Neumann claims that blood evaporated at a temperature of 60° F. gives a residue exhibiting certain appearances called "blood pictures," which are characteristic of different animals. (Die Erkennung des Blutes, Leipzig, 1869.)

There are two marked points of difference, under microscopic examination, between mammalian and oviparous blood: 1. The circular outline of the corpuscles of the former, in contradistinction to the oval shape of the latter; 2. The presence of a nucleus in oviparous, and its absence in mammalian, blood. Whether the stain be fresh or dry, recent or very old, these differences are apparent and unmistakable. Many instances have occurred in murder trials in which the defendant has claimed that stains on his clothing were produced by the killing of a fowl or were due to the blood of fishes. In all such cases, the falsehood, if it be one, is easily proved.

The possibility of distinction between the blood of man and the other mammalia has been claimed by several eminent authorities, including Schmidt, Richardson, Formad, Wormley, and Reese, and strenuously denied by others, especially by Woodward and Ewell. Nearly all writers on medical jurisprudence express grave doubts as to the value of opinions based upon variable fractional differences obtained by comparative measurements of corpuscles derived from dried stains.

The blood-corpuscles of most of the mammalia are smaller than those of man, so that when human blood is compared with that of the ox, for example, each being in a fresh state, by examining the specimens side by side upon the same slide (for an account of Richardson's method of preparing such slides, see American Naturalist, May, 1876) a difference in the diameters of the two is, even to the most careless observer, clear and unmistakable.

The diameters of the corpuscles of a large number of animals have been measured and their averages recorded. Suppose now that a specimen of mammalian blood of unknown origin and in a dried state upon cloth is subjected to microscopic examination: within what limits is the observer warranted in expressing his opinion as to its origin?

In answer to this inquiry, we remark that there is no fixed or invariable average for the diameters of the blood-corpuscles of any animal. It is true that the average does not vary beyond certain limits, but these limits have been shown by the observations of Woodward and Ewell upon the corpuscles of man and the dog to be much greater than formerly supposed. Not only is this true, but the averages given by different observers, while agreeing closely in some instances, show considerable differences in others, so that, assuming the principle involved in this method of comparison to be correct, "it entirely depends," as Formad observes, "upon whose figures we accept whether we can or cannot discriminate between human blood and the blood of certain other animals."

The following tables give the average measurements of the bloodcorpuscles of man, dog, guinea-pig, rabbit, rat, mouse, opossum, wolf,

ass, pig, ox, cat, horse, sheep, and goat, according to different observers. The figures represent fractions of the inch, and the measurements are of fresh blood dried upon glass.

The subjoined diagram represents the apparent areas of the corpuscles of these animals (except the ass and wolf) according to the tables. The outside columns represent respectively the highest and the lowest

.1-6189

.1-6366

.1-6369

average of any observer, while the middle column represents the averages of the averages. It is obvious that the blood of some of the common animals cannot be distinguished from that of man even in a fresh state.

Formad (Jour. of Comp. Med., 1888, vol. ix., p. 269) attacks the measurements of Woodward, and endeavors to prove that they are erroneous. To this it may be replied that if it be possible that so distinguished a microscopist as

Woodward did make an error, it demonstrates that the conclusions drawn from measurements of a less competent observer would be of little value as evidence in a capital trial.

There is no doubt, however, that micrometric measurements are liable to error. In order to ascertain the relative accuracy of such determinations made by different competent observers, Ewell ruled a glass slide with fifteen lines, making spaces approximately of 1-250 to 1-125 of an inch, and caused the same to be measured by six well-known microscopists, who were instructed to take the mean of at least five measurements of each space. Using standard micrometers by the same maker, the result showed that the measurements of the same space by different observers varied from 0 to 1-9090. This is a greater difference than that between the average diameters of the blood-corpuscles of man and any of the common domestic animals except the sheep and goat, according to all observers. On the other hand, Wormley and two others measured ten lines of the 1-1000 divisions of a stage micrometer, and it was found that they agreed in the several readings within 1-200,000 of an inch.

3200

Man.

Fig. 19. Diagram representing the Comparative Sizes of the Red Blood-Corpuscles of various Animals. (1450 diameters.)

The uncertainty of discrimination between human blood and that of other mammals does not.depend upon possible errors of measurement.

Gulliver, referring to his own extensive measurements of corpuscles, says: "Special circumstances, too, of which we have not yet sufficient knowledge, may affect the value of any series of such measurements as are recorded in these tables. When the bird is much excited and the circulation quickened by attempts at its capture in an aviary, the oval figure of its red blood-corpuscles may be more elongated than in the same bird when quietly at rest; . . . and my attention was sometimes arrested by like diversions in other vertebrates at different times and seasons, though not in so many observations, and with such notes as would be needful for satisfactory conclusions. But the facts are sufficient to show that exact and extensive investigations are yet necessary on the comparative magnitude of the red corpuscles and their aggregate proportion to the other parts of the blood at different seasons and under different circumstances. For example: whether minute diversities in the corpuscles may not be found in man at the tropics and at the frigid zone; in animals at rest and during violent exertion; in hibernating animals during summer and winter; in species subject to periodic changes in temperature." (Proc. Zoolog. Soc., London, 1875, p. 477.)

Woodward measured 1766 corpuscles in groups of 22 to 140 upon twenty-two photographic negatives taken from the blood of eight persons, and found averages as follows:

*

General average of 1766 corpuscles, 1–3090.

(Trans. Am. Med. Asso., 1876, vol. xxvii., p. 303.)

In another series of measurements of 651 corpuscles in groups of 50 each Woodward found the averages to be:

In the dog Woodward reports the averages obtained from 1571 corpuscles measured upon thirteen negatives:

* In these and the succeeding values the original decimal measurements have been reduced to the form of vulgar fractions, and the order of statement slightly transposed.