less complete, to the perfectly free transmission of the rays. There are many objects of great delicacy, in which the "diffraction-band" is liable to be mistaken for the indication of an actual substance; on the other hand, the presence of an actual substance of extreme transparency, may sometimes be doubted or denied, through its being erroneously attributed to the "diffraction-band." No rules can be given for the avoidance of such errors, since they can only be escaped by the discriminative power which education and habit confer. The practised Microscopist, indeed, almost instinctively makes the requisite allowance for diffraction; and seldom finds himself embarrassed by it, in the interpretation of the visual appearances which he obtains through a good instrument. Besides this unavoidable result of the inflection of the rays of light, there is a peculiar phenomenon attendant upon oblique illumination at certain angles in one direction; which consists in the production of a double image, or a kind of overlying shadow, sometimes presenting markings equally distinct with those of the object itself. This image, which is not unlike the secondary spectrum formed by reflection from the outer surface of a silvered-glass mirror, has been called the "diffracting spectrum;" but its origin does not really lie in the diffraction of the luminous rays, since on the one hand it cannot be explained according to the laws of diffraction, and on the other it may be traced to an entirely different cause. An object thus illuminated is seen by two different sets of rays; those, namely, of transmitted light, which pass through it obliquely from the source of the illumination to the opposite side of the object-glass; and those of radiated light, which, being intercepted by the object, are given off from it again in all directions. (The latter alone are the rays whereby the images are formed in any kind of "black-ground" illumination, §§ 61, 62.) Two different images will be formed, when the illuminating pencil is very oblique, and the angular aperture of the objectglass is wide; one of them by the light transmitted to one extreme of its aperture, the other by the light radiated to its general surface; and one or the other of these images may be stopped out, by covering that portion of the lens which receives, or that which does not receive, the transmitted pencil. This "diffracting-spectrum" may be produced at pleasure, in an object illuminated by direct light and seen with a large aperture, by holding a needle or a horse-hair before the front lens, so as to split the aperture into two parts.

97. Errors of interpretation arising from the imperfection of the Focal adjustment, are not at all uncommon amongst young

1 Thus the account given by Prof. Sharpey and the Author, of the structure of Muscular Fibre (Chap. XVIII), has been called in question by observers who had not seen their preparations, on the ground that the "diffraction-band" had not been allowed for. To whatever the appearance in question (Fig. 326) may be due, there cannot be the slightest question that it does not arise from diffraction.

Microscopists. With lenses of high power, and especially with those of large angular aperture, it very seldom happens that all the parts of an object, however small and flat it may be, can be in focus together; and hence the focal adjustment being exactly made for one part, everything that is not in exact focus is not only more or less indistinct, but is often wrongly represented. The indistinctness of outline will sometimes present the appearance of a pellucid border, which, like the diffraction-band, may be mistaken for actual substance. But the most common error is that which is produced by the reversal of the lights and shadows, resulting from the refractive powers of the object itself; thus, the bi-concavity of the blood-disks of Human (and other Mammalian) blood, occasions their centres to appear dark, when in the focus of the Microscope, through the dispersion of the light which it occasions; but when they are brought a little within the focus, by a slight approximation of the object-glass, the centres appear brighter than the peripheral parts of the disks (Fig. 315). The same reversal presents itself in the case of the markings of the Diatomacea; for these, when the surface is exactly in focus, are seen as light hexagonal spaces, separated by dark partitions; and yet, when the surface is slightly beyond the focus, the hexagonal area are dark, and the intervening partitions light (Fig. 80). The best means of avoiding errors of interpretation arising from this source, lies in the employment of the lowest powers with which the particular structures can be distinguished; since, if the different parts of the surface and margin of the object can be simultaneously brought so nearly into focus that a distinct view may be gained of all of them at once, no false appearances will be produced, and everything will be seen in its real aspect.

98. A very important and very frequent source of error, which sometimes operates even on experienced Microscopists, lies in the refractive influence exerted by certain peculiarities in the form or constitution of objects, upon the rays of light transmitted through them; this influence being of a nature to give rise to appearances in the image, which suggest to the observer an idea of their cause that may be altogether different from the reality. A very characteristic illustration of the fallacy resulting from external configuration, is furnished by the notion which long prevailed among Microscopic observers, and which still lingers in the public mind, of the tubular structure of the Human hair. This notion has no other foundation, than the existence of a bright band down the axis of the hair, which is due to the convergence of the rays of light occasioned by the convexity of its surface, and which is equally shown by any other transparent cylinder; and it is unmistakably disproved by the appearances presented by thin transverse sections of Hair, which show that it is not only filled up to its centre with a medullary substance, but that its centre is sometimes even darker than the surrounding

part (Fig. 311). Of the fallacy which may sometimes arise from diversities in the refractive power of the internal parts of an object, we have an equally "pregnant instance" in the misinterpretation of the nature of the lacunae and canaliculi of Bone (Fig. 300), which were long supposed to be solid corpuscles with radiating filaments of peculiar opacity, instead of being, as is now universally admitted, minute chambers with diverging passages, excavated in the solid osseous substance. For just as the convexity of its surfaces will cause a transparent cylinder to show a bright axial band, so will the concavity of the internal surfaces of the cavities or tubes hollowed out in the midst of highly refracting substances, occasion a divergence of the rays passing through them, and consequently render them so dark that they are easily mistaken for opaque solids. That such is the case with the so-called "bone-corpuscles," is shown by the effects of the infiltration of Canada balsam through the osseous substance; for when this fills up the excavations,-being nearly of the same refractive power with the bone itself, and being also quite transparent, and (in thin lamina) quite colorless,-it obliterates them altogether. So, again, if a person who is unaccustomed to the use of the microscope should chance to have his attention directed to a preparation mounted in liquid or in balsam, that might chance to contain air-bubbles, he will be almost certain to be so much more strongly impressed by the appearance of these, than by that of the object, that his first remark will be upon the number of strange-looking black rings which he sees, and his first inquiry will be in regard to their meaning.

99. No experienced Microscopist could now be led astray by such obvious fallacies as those alluded to; but it is necessary to dwell upon them, as warnings to those who have still to go through the same education. The best method of learning to appreciate the class of appearances in question, is the comparison of the aspect of globules of Oil in water, with that of globules of Water in oil, or of bubbles of Air in water or Canada-balsam. This comparison may be very readily made by shaking up some oil with water to which a little gum has been added, so as to form an emulsion; or by simply placing a drop of oil of turpentine and a drop of water together on a slip of glass, laying a thin glass cover upon them, and then moving the cover several times backwards and forwards upon the slide. Now when such a mixture is examined with a sufficiently high magnifying power, all the globules present nearly the same appearance, namely, dark margins with bright centres; but when the test of alteration of the focus is applied to them, the difference is at once revealed; for whilst the globules of Oil surrounded by water become darker as the object glass is depressed, and lighter as it is raised, those of

If this latter mode be adopted, it is preferable, as suggested by the authors of the "Micrographic Dictionary" (Introduction, p. xxxii), to color the oil of turpentine with alkanet, or some similar substance, for its more ready distinction.

Water surrounded by oil become more luminous as the objectglass is depressed, and darker as it is raised. The reason of this lies in the fact, that the high refracting power of the oil causes each of its globules to act like a double-convex lens of very short focus; and as this will bring the rays which pass through it into convergence above the globule (i. e. between the globule and the objective), its brightest image is given, when the object glass is removed somewhat further from it than the exact focal distance of the object. On the other hand, the globule of water in oil, or the minute bubble of air in water or balsam, acts, in virtue of its inferior refractive power, like a double-concave lens; and as the rays of this diverge from a virtual focus below the globule (i. e. between the globule and the mirror), the spot of greatest luminosity will be found, by causing the object-glass to approach within the proper focus. Now in the "protoplasm" of the cells of the lower Plants, and in the "sarcode" of the lower animals, oil-particles and vacuoles (or void spaces) are often interspersed; and present, at first sight, so very striking a resemblance, that the inexperienced observer may well be pardoned for mistaking the "vacuoles" for larger globules of a material more refractive than the gelatinous substance around them. But the difference in the effects of alterations of focus on the two sets of appearances, at once serves to make evident the difference of their causes; and this, moreover, is made obvious by the effect of oblique light, which will cause the strongest shadow to exhibit itself on opposite sides, in the two cases respectively. It will be obvious that minute elevations and depressions of the surface of the object will exert an influence upon the course of the rays which it transmits, very similar to that which proceeds from the presence of globular spaces, filled with transparent substances of greater or less refracting power, in its interior; and that the discrimination between the two may be made by the same means. For if the dots appear more luminous as the object-glass is raised, and darker as it is depressed, they may be interpreted as being due to convexities upon the surface; but if the contrary is the case, they may be referred to concavities.

100. Among the sources of fallacy by which the young Microscopist is liable to be misled, one of the most curious is the Moleeular Movement which is exhibited by the particles of nearly all bodies that are sufficiently finely divided, when suspended in water or other fluids. This movement was first observed in the fine granular particles, which exist in great abundance in the contents of the pollen-grains of plants (sometimes termed the forilla), and which are set free by crushing these grains; and it was imagined that they indicated the possession of some special vital endowment of these particles, analogous to that of the spernatozoa of animals. In the year 1827, however, it was announced by Dr. Robert Brown, that numerous other substances, organic and inorganic, when reduced to a state of equally minute

division, exhibit a like movement, so that it cannot be regarded as indicative of any endowment peculiar to the fovilla-granules; and subsequent researches have shown, that there is no known exception to the rule, that such motion takes place in the particles of all substances, though some require to be more finely divided than others, before they will exhibit it. Nothing is better adapted to show it, than a minute portion of gamboge, indigo, or carmine, rubbed up with water; for the particles of these substances, which are not dissolved, but only suspended, are of sufficiently large size to be easily distinguished with a magnifying power of 250 diameters, and are seen to be in perpetual locomotion. Their movement is chiefly of an oscillatory kind; but they also rotate backwards and forwards upon their axes, and they gradually change their places in the field of view. It may be observed that the movement of the smallest particles is the most energetic, and that the largest are quite motionless, whilst those of intermediate size move, but with comparative inertness. The movement is not due (as some have imagined) to evaporation of the liquid; for it continues, without the least abatement of energy, in a drop of aqueous fluid that is completely surrounded by oil, and is therefore cut off from all possibility of evaporization; and it has been known to continue for many years, in a small quantity of fluid enclosed between two glasses in an air-tight case. It is, however, greatly accelerated, and rendered more energetic, by Heat; and this seems to show that it is due, either directly to some calorical changes continually taking place in the fluid, or to some obscure chemical action between the solid particles and the fluid, which is indirectly promoted by heat. It is curious that the closer the conformity between the specific gravity of the solid particles and that of the liquid, the less minute need be that reduction in their size which is a necessary condition of their movement; and it is from this that the substances just named are so favorable for the exhibition of it. On the other hand, the particles of metals, which are from seven to twelve times as heavy as water, require to be reduced to a minuteness many times greater than that of the particles of carmine or gamboge, before they become subject to this curious action. In any case in which the motions of very minute particles, of whatever kind, are in question, it is necessary to make allowance for this "molecular movement;" and the young Microscopist will therefore do well to familiarize himself with its ordinary characters, by the careful observation of it in such cases as those just named, and in any others in which he may meet with it.

101. Comparative Values of Object-Glasses; Test Objects.-In estimating the comparative values of different object-glasses, regard must always be had to the purpose for which each is designed; since it is impossible to construct a combination, which shall be equally serviceable for every requirement. It is commonly assumed, that an Objective which will show certain test