Imagens das páginas

Figs. 11-11. Surface views of epidermis of Tradcacantia discolor, from first appearance

of stomata to maturity of the Mime.
Fig. 15. Cross; section of ubout tlie a^oof Xo. 12, b.
Fig. 16. Cross section of the age of Xo. 13.
F^g. 17. Cross section of full-grown.
Fig. 18. Longitudinal section of t!ie same.

X WO- The arrow inilic itn the dinvl'o-.i of the point of t'.ic leaf.



The igneous scenes of the prairies have become very common place in description. But where is there tiny account, either scien title or popular, of the interesting aqueous phenomena, in winter ami in summer?

How it may be in the region protected on the north by the Lake Superior highlands and affected by the air of the lakes, the writer does not know, except that the temperature is much modified. But in central Iowa intense cold is of frequent occurrence, and there are conditions along with it that often bring out the splendors and wonders which we associate with arctic scenes. Parhelia, or mock suns, at morning or evening, are common ; without exaggeration it may be said that they equal the real sun in brilliancy, and arc indeed blinding to the sight. After witnessing them, an eastern man regards all that he has seen of this phenomenon at the east as insignificant. So likewise, mock moons, and both lunar and solar halos, crosses, and far-extending complicated circles of light, with blight spots at the intersections, may be mentioned as sights by no means unusual, and often of great magnificence and duration, continuing a good part of the day or night. The writer remembers, for example, a circle passing through the sun and reaching horizontally quite around the sky, making part of a cross inscribed within another circle around the sun, there being also four tangent circles at the ends of the cross; and this was visible for several hours before and after midday. The cloudless sky of the West conspires with spicules of frozen vapor, to render these effects not rare; for the West, at least' beyond the vicinity of the lakes, is bright and sunny at all seasons.

Feathery crystals, frequently of great size and beauty, and completely clothing every exposed object, are sometimes to be seen for a succession of" mornings and would number many repetitions in the course of even7 winter. The writer counted a dozen fine exhibitions of this kind before one winter was half gone. In cold weather, a perceptible thin vapor comes on at night, not uncommonly, when the air has a stillness favorable to the growth of this icy leafage. In certain covered situations, where moisture rises, cobwebs are changed to a lace-work of crystals, the length and delicacy of which would be incredible to one who never lived in such a climate. In a cellar stair way, the plastering and shelf and every article on it were soon robed with a polar-bear fur of icy filaments, so long that the smaller articles lost their identity of shape.

It is well known that the fine porous soil of the West has a marvellous ability to support vegetation, during the long droughts that characterize the region. The cracking of the earth in a prolonged drought is wonderful, especially along the beaten surface of roads. Fissures over an inch across have been measured. How the subsoil can retain any moisture, with such openings down into its heart, is a mystery. On the high treeless rolling prairie, however, at the summit level between river systems, water is readily obtained at a depth of from fifteen to twenty feet, though not always in sufficient quantity. The manner in which wells are made for the supply of mills, in such a situation, is worthy of publication. A shaft is sunk, say thirty feet, and from the bottom galleries are drifted in various directions, in the style of a mine, sometimes to the length of a hundred feet. Thus, numerous very small veins are struck, which, all together, give a large supply of water. The workmen report these veins as occurring at somewhat regular intervals, and as indicated by a root-like mass of darker earth; it is affirmed, too, that they follow one general direction,— in. one instance at least, said to be transverse to the surface drainage.

In this connection, reference may be made to the subterranean cryptogams that penetrate almost every inch of the deep, loamy gray clay beneath the top soil in that prairie region, and perhaps in all similar districts. This vegetation, threadlike or coarse stringlike, is coated with dark discolored earth, and is mostly dead, the thread lying shrivelled, black and loose in its cylindrical cavity ; but the writer has found the filaments apparently fresh and living at a great depth — even to the depth of eight feet, if his memory is not at fault.

One very common peculiarity of the surface drainage may be

noticed —the extent to which the water of the sloughs, or swales, reaches up the acclivities on either hand, even where the interval has a very considerable descent in the line of flow ; there is thus a broad concave bog that must strike a stranger with surprise, for it is not due to springs, but rather to a spongy retention of rainfall. Some peculiarities of prairie storms should not be omitted in this sketch. Nothing at the West is done by halves; when it rains, it rains; and the general surface is so uniform, the soil to a certain depth so pervious, that something like a subterranean lake is suddenly formed, which rapidly rises, flooding cellars and even bursting up the cement of cellar floors by hydrostatic pressure, if cement has been resorted to. by the trustful immigrant.

One species of prairie storm should be elevated to the rank of a genus. It is mostly nocturnal in its habits and prowls all night; its distinguishing characters are surges of rain, rhythmic roar of wind like that of heavy billows on a coast, incessant quiver of lightning, and overlapping continuous peals of thunder. It is as if the spirits of the old American Mediterranean sea were claiming again their last battle-ground — a suggestion harmonizing with the. ocean-like level of the country and the looming mirages of sunny days. But the lightning of this species of storm seems to be among the clouds, and the new-comer soon becomes fearless; indeed, it does not require a long residence at the West to make one familiar with lightning, however timid he may have been at the East, although it remains true that thunder gusts are not pleasant to a person who is out on the open prairie, where man or horse is the only prominent object to attract the downward or upward bolts of electricity. Finally there is something peculiarly giand in western thunder. No hills break its smooth roll, and its long crescendos and diminuendos give a breath and cadence to the sound, as if chariots could be heard rolling on for hundreds of miles oi-er the level prairie floors.

The subjects of this article have not been in the path of the writer's special study; but he believes that the prairie region offers a fresh and interesting field of observation in this regard. The reports and hooks where the information might properly be looked for, have failed to give him any information in respect to the relative humidity of the prairie atmosphere — a matter of prime importance. On average winter days, the writer found it from forty to fifty hundredths of saturation.


BY It. H. WARD, M.D.*

The nominal focal length of an achromatic objective, as used by microscopists generally, represents its amplifying power as actually used in the compound microscope. Even the equivalency in amplifying power with a single lens of the same focus is no longer distinctly realized, while the size and appearance of the combination, its working focus, angular aperture, and microscopical efficiency, are not even hinted by the figures used. The nominal focus represents the magnifying power and those properties dependent on it. Like other measurements, these must be stated by comparison with known standards. To use diverse and unknown units of measurement in cases designed to be compared with each other is simply self stultification. To call two lenses, of identical magnifying power, respectively one-fourth and onesixth inch lenses, is just as indefensible as to call two houses of equal height, forty and fifty feet high respectively. To argue against the existing looseness of usage in naming lenses, is only to state what everybody knows in regard to the advantages of uniform standards of measurement generally. So impressed are many microscopists with the urgency of this question, and so determined are they to escape from some of the present confusion, that a committee has been appointed to report on the subject. Though that committee is unprepared to report, it is believed that giving publicity to some facts and opinions involved in the consideration, may lead to useful agitation and to increased defiuiteness of ideas and of information in regard to it. Of course it would be premature to claim or expect accuracy of statement or safety of opinion in such a complication of disputed questions; and what is said, is designed to be contributory and suggestive, and in no degree dogmatic or final.

The great variation in objectives of identical name is familiarly

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known aud is undisputed. Among other people one-fourth of an inch is less than four-tenths and more than one-fifth ; but among microscopists it may often be more than the first or less than the last. An indefinite number of figures might lie published to prove or illustrate this irregularity, the writer having been particularly interested in making and recording these comparisons for more than a dozen years, and Messrs. Bicknell, Biscoe, Higgius, Cross, aud many others having been especially interested in the same study ; but it is idle to prove what everybody knows and admits. So familiar have some of these apparent errors become by use, and good usage too, that they have been often accepted as established, even one of the latest authorities* stating the power of the one-fourth-iuch objective five times as high as that of the oneinch

In the early days of the compound microscope as a really useful instrument, we find microscopists wishing that microscope makers would '-grind their glasses to some settled standard."! We are willing to be more reasonable now, or else the conditions stated have become more difficult. We do not desire, nor consider it practicable that the opticians should make all their combinations of certain definite and conveniently graded powers ; but we do propose so to name our powers, if we can, that each number shall group together ail those powers of which it is the nearest aud best description.

Makers would doubtless be considered as doing a favor to those who use their instruments if they would, after finishing lenses, carefully estimate their powers and name them by the fractions most nearly representing those powers. But even if this were done, and much more now when this is certainly not done, or not done upon such a uniform plan as to be satisfactory, microscopists should always reexamine their lenses in order to be definitely informed in regard to one of their most important properties.

The easiest method of examining the magnifying power of an objective, by measuring the image (of a known object) which it forms at a standard distance (now ten inches), was as well understood a hundred years ago as now; a lattice of fine silver wire or of human hair, or a scale ruled on glass, being used to measure

* Suffolk, Microscopical Manipulation, London. 1S70. t Balur on Microcodes. London. 1712.

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