and hence we have it in our power to measure the extreme limits at which the object continues to be visible.

For the formation of the dioptric images achromatic object-glasses might be used; but even where those of the shortest focal length are employed, the object whose image it is required to form must be placed at a great distance. This would cause various difficulties, and only be practicable with a microscope placed horizontally-unless, indeed, the object selected were very minute, in which case the accurate determination of its diameter (from which that of its image must be afterwards deduced) would be rendered difficult.

Small air-bells in a fluid are for this purpose far better. I employ by preference a watery solution of powdered gum arabic, which always contains numbers of such air-bells originating in the air entangled among the particles of the powder. The water employed should have stood for a considerable time freely exposed to the air, or been shaken up with the air for some time; for when we use water which is not saturated with air, the bubbles in the fluid gradually become smaller, and images formed in them decreasing in magnitude, cause errors in the subsequent measurements, as we shall actually find to be the case.

A drop of the fluid must then be placed on a clean glass objectslide, and covered with a good clear mica plate, a ring shaped piece being interposed, in order to prevent the flattening of the air-bells by pressure. The object-slide is then placed under the object-glass upon the stage of the microscope, and an air-bell of suitable size for the formation of the images is sought for. All do not give images of the same degree of sharpness; a peculiarity dependent on the fact that some air-bells are in contact with the covering-plate, and consequently have their spherical form disturbed to some extent, or on the presence of small molecules in the fluid above or beneath the air-bell, or even in its interior, causing some haziness of the image, just as defective polish of a glass lens would do. It will, however, be always easy to find some which will form images of the utmost distinctness and purity.* This may be ascertained in the first instance by holding between the mirror and stage some easily recognized object, e. g. a piece of paper or the like. The image is always formed on the under surface of the air-bell, which must consequently be brought nearer to the object-glass than when it is desired to bring its margins into focus.

The object whose image is to be the subject of examination should be placed upon an apparatus, which can be moved upward and downward in the space between the mirror and stage. In some microscopes this can hardly be done, either from the space being too limited, or in consequence of the drum-like form of the foot of the microscope which quite envelops the space. If such microscopes, in place of a mirror, be provided with a reflecting prism, the object may be placed opposite the side external to the microscope. The instruments best adapted for

=

The following example will demonstrate this. I brought a printed page of a book to such a distance from an air-bell that the length of the image of the whole page was 1-7th millimetre about 1-180th of an inch, and that of the image of each letter about 1-480th millim 1-12.000th of an inch. In spite of their minuteness, these images, formed by reflected light, possessed such clearness and sharpness, that under a magnifying power of 154 diameters the whole page was without difficulty legible.

the manipulation which we are describing are, however, those whose illuminating apparatus consists of a mirror and converging lens, which can be shifted up or down. The lens being removed from the ring which supports it, the object is substituted in its place. The relative magnitudes of object and air-bell must be such that the image shall be exceedingly minute when the object is tolerably near to the stage. On afterwards increasing the distance between the object and air-bell, it is not difficult to find the limit at which the image (under a given magnifying power) is barely visible.

Of course it is impossible to measure directly the dimensions of this most minute visible image, for our best micrometric methods will here be found of no avail. Yet their size may be estimated with extreme accuracy in the following manner. At the same distance from the airbell and in place of the object used, substitute another body, such as a piece of card, of 4 to 5 centimetres = 13ths to 2 inches diameter, which has been exactly measured. Let this be now again measured (by some of the micrometric methods elsewhere alluded to*), just as if it were a real object. By dividing the real diameter by the apparent diameter, the amount of diminution is found; and this is the same for all objects at a like distance from the air-bell. to do, in order to find the amount of more minute object, but to divide its pressing the diminishing power.

We have, consequently, nothing diminution of the image of the true diameter by the figure ex

969

For example, let the true diameter of the greater object be 5 centimetres to 1969 English inches, and the diameter of its image= 32.2 micromillimetres,† 00127 English inches, then the figure expressing the amount of diminution will be 19 1553 very nearly. If now the smaller object have a diameter of 175 micromillimetres = 00689 English inches, then must its image at the limit of vision be in diameter 0000044, or about 25000th of an English inch. When exact micrometric methods are employed, it is easy in this way to estimate the diameter of an image even to millionth parts of a millimetre, i. e. to 25,400,000th parts of an inch.

As for the object suitable for these investigations, it is plain that we have an extensive choice. To find the limit of vision for bodies of a round or long thread-like form, grains of pearl sago, or vegetable bodies, such as mustard-seed or the pollen-granules of many plants, hairs of animals, metallic wires, &c., may be employed. Small round openings and chinks may serve for the determination of the visibility of pos itive images of light. In the last case, care must of course be taken, by means of suitable screens, to shut off all light except what passes through the aperture. To determine the defining power, metallic wire gause is a suitable object, or two holes placed near each other in a black metallic plate. The images of such objects resemble exactly a double star viewed through a telescope (kijker). The bodies may likewise be placed in different circumstances in order to ascertain the influence of these upon the limits of vision. Thus we may use as an object a very

* See translation from Het Mikroskoop in Monthly Journal of Medical Science, June, 1852. p. 453, et seq.

+The micromillimetre is equal 1-1000th millimetres See Monthly Journal, June, 1852, p. 456.

0000394 English inches.

thin glass capillary-tube placed in water, and compare it with tender organic tubes and vessels, which may also be seen in water, but whose limit of visibility is of course far more circumscribed than that of absolutely opaque objects.

In fact this method admits of innumerable variations, and is consequently of most extensive application. Besides, when proper precautions are taken, it gives results perfectly sure and comparable. Especial care is however requisite in the mode of illumination. For it is certain, that when the field has a clear white ground, the contrast causes minute opaque bodies (i. e. objects which are dark by transmitted light) to continue visible, which against a grayish or light-blue background could not be seen. Hence it is by no means indifferent to receive on the mirror light from a white cloud, from a dull overcast, or clear blue sky. Artificial light cannot be used in these experiments, for the image of the flame becomes diminished like the object, and hence a clear field of view is not to be obtained. The observations must consequently be made by daylight; and whenever comparable results are sought for, the mirror should always be directed to the clear, blue, cloudless sky-this being a distinct atmospheric condition to which others in similar circumstances may refer in conducting the same experiment. The mode of ascertaining the limit of vision, with a given amount of illumination, may be gathered from different examples in the body of this work. It will likewise be found that for all such observations, even when the highest magnifying powers are employed, the flat mirror is perfectly sufficient, since in the image in the field of view formed by the air-bell, all the rays proceeding from the mirror are united and constitute an object of considerable luminous intensity.

8. On an Instrument for taking Soundings; by F. MAXWELL LYTE, Esq., (Phil. Mag., [4] vi, 344.)—I see, from what Dr. Scoresby has brought before the Association at Hull, that there seems to be some dif. ficulty about obtaining correct soundings in places where the currents are strong and flow in different directions at the different points of depth, causing the line to assume different curves in its descent; and when it comes to be measured over, after the weight has reached the bottom and been hauled up again, the measurement gives no approximate idea of the real depth. Now it is plain that this mensuration of the depth of water might be as well made by estimating its vertical pressure, as, in measuring the height of mountains, we measure the barometrical pressure of the air; and so I would propose to do it by an instrument constructed as follows:

An accurately constructed tube of gun-metal or brass, or some metal not very easily corrodible by salt water, has a glass tube fitted on to it on the top by a screw joint, and again on the top of the glass tube is fitted a strong hollow copper ball by a similar screw joint. The lower tube, which we will call a, has a well-turned piston fitted to it, from which runs a rod which is only a trifle longer than the tube a, and just enters the tube b when the piston is at its lowest point. A well-made spring is placed in the tube a above the piston, and the tube a being narrowed at the top, so as just to admit the free passage of the rod, and the rod having a little button at its top, the piston is kept at its lowest point by the spring, except when sufficient pressure is applied from below

to compress the spring. The glass tube has a small ring fixed in it, just so as to stick at any point to which it is pushed, and the button at the top of the rod serves to push the ring straight, and the ring thus forms an index of the degree to which the spring has been compressed. The ball on the top serves as a mere reservoir of air to equalize the action of the apparatus as much as possible. The whole of this apparatus is enclosed in a wire cage for the sake of protection from blows. To graduate this apparatus, I let it down in a known depth of water, say ten fathoms, and having observed the point to which the ring in the glass tube is pushed, and having marked this point off, the ball is to be unscrewed, and with a small ramrod the ring is to be pushed down till it rests on the top of the piston rod. The ball being replaced, the apparatus is sunk in twenty fathoms; after a similar manner it is sunk in thirty, and next in forty fathoms. This will test the accuracy of the apparatus; and the marks made on the glass tube b after each trial will give a scale from which the whole tube may be graduated, even to thousands of fathoms, if the tube be long enough or the spring strong enough. I have been induced to make this communication on account of the great use which may be made of such an apparatus.

9. Louis Samann.—Mr. Sæmann, who formerly travelled through the U. States, has arrangements at Paris for the sale of specimens or collections in Mineralogy, Geology, and Paleontology. To a thorough knowledge of minerals, Mr. S. unites a most excellent and obliging disposition; and his establishment is one of the largest in Europe. His address is Louis Sæmann, Comptoir Mineralogique et Palæontolo gique, Rue St. Andrè-des-Arts, No. 45, Paris.

10. Cabinet of Minerals for sale.--A large and excellent cabinet of minerals, and mineralogical works accompanying, is for sale at Washington. Address Fr. Markoe, Esq., Washington City. It is one of the best private collections in the country.

ence.

11. OBITUARY.—JAMES E. TESCHEMACHER died suddenly near Boston on the 9th of last November. Mr. Teschemacher, although engrossed with other cares, has been an unceasing and successful laborer in SciHe was an exact observer, and delighted in searching out with his microscope what passed unnoticed by others. He has contributed much to our knowledge of American minerals, both through the detec tion of rare forms of species and by his publications. A letter from him written but two days before his death, states his plans for research through the winter, and the progress he had already made in his chemical examination of the Chesterfield pyrochlore. He was most generous in communicating the results of his researches and allowing the free use of his Cabinet of Minerals to those who appreciated its excellencies. They were mostly miniature specimens, but often of rare interest. He has been laboring of late on the subject of the Fossils connected with coal, and the structure of the coal itself, and had collected much that was novel, which he was preparing for publication.

12. Die Kreidebildungen von Texas, und irhe organischen Einschlüsse; Von Dr. FERD. ROEMER. 410. Bonn, 1852.-This work has already been briefly noticed in the September number of this Journal. The Introduction contains, 1. A sketch of the geographical situation and

general orographic condition of Texas; 2. General geognostic constitution of the country; 3. Diluvial and alluvial formations; 4. Tertiary formations; 5. Older or Paleozoic strata; and 6. Plutonic rocks.

The principal part of the volume is devoted to a geological description of the chalk formation of Texas, with an enumeration and description of its organic remains, which occupy eighty-eight pages of the volume. These are followed by descriptions of Palæozoic fossils, and of three species of fossil wood from the tertiary.

The cretaceous fossils figured occupy ten quarto plates, and number one hundred and twenty-four. Of these one hundred and one are new or yet undetermined species, and twenty-three are identical with species previously known. These are beautifully illustrated. The Paleozoic fossils number ten species, of which eight are new. These are chiefly of the carboniferous period, and we recognize them as forms which prevail farther to the north and west in the same formation. The three species of fossil wood from the tertiary, described by Unger are Sillimania Texana, Roemeria Americana, and Thuyoxylon Americanum.

The fossils here described and figured, had been already indicated in Dr. Roemer's previous work on Texas.*

The present work offers a very valuable accession to our knowledge of the American Cretaceous formation; contributing more species of fossils than have been published since the appearance of Dr. Morton's Synopsis in 1830. Dr. Roemer has given some valuable observations on the climatic influences upon the fauna of the chalk period, and has instituted comparisons between this formation in Texas and other parts of America, as well as with the same formation in Europe.

On comparing these figures of Texan cretaceous species with collections from Nebraska, a few degrees farther north, we are struck with their almost total dissimilarity. In the little which we already know of it, we have but a foreshadowing of what is yet in store for us when this formation, which extends from the Tropic of Cancer to the 48th degree of latitude, shall have been completely explored.

II.

13. Geological Map of Keweenaw Point, Lake Superior, Michigan ; by J. D. WHITNEY, assisted by S. W. HILL and W. H. STEVENS.This is a large pocket map of the Lake Superior Mining Region, 2 feet by 4 in its dimensions. It gives an admirable view of the Geological structure of the region, and is excellent in illustration of an article in this volume from the Report of Messrs. Whitney and Foster. The different rocks are indicated as usual by colors, and many facts of great geological interest are indicated by their arrangement, and the positions of the various metallic veins which intersect them. The map contains also a section across Keweenaw Point by Copper Falls and Northwestern Mines. It is invaluable to geological science as well as to the topographer and traveller.

14. People's Journal; Vol. I, Nos. 1 and 2, November and December, 1853. 32 pp. large 8vo.-This new popular monthly opens with an article on Willison's Hand Thrashing Machine. The Journal is de

*Texas: mit besonderer Rucksicht auf deutsche Auswanderung und die physicalischen Verhältnisse des Landes nach eigener Beobachtung geschildert; von Ferdinand Roemer. Mit einen naturwissenschaftlichen Anhange und einer topographische-geognostischen Karte von Texas. Bonn, bei A. Marcus, 1849.