Изображения страниц
PDF
EPUB

from within. If the condensing lens is used, I do not think any material advantage can be obtained from the prism, and its expense is a decided objection. In the winter season, in this latitude, a prism of over five inches hypothenuse is required, and its cost is a serious item. An ordinary glass mirror answers, I think, quite as well for the tissues and most other purposes. There are, however, certain objects, such as the Pleurosigmata and some other diatoms, the Nobert's test-plate, and the scales of certain insects, for which the condensing lens is unnecessary. The achromatic condenser, illuminated by a parallel solar pencil, answers better in these cases, and if it is properly managed no diffraction or interference phenomena are produced. I am satisfied that in such cases the pure parallel pencil obtained from the prism gives better definition to the image than can be obtained by the double pencil reflected from an ordinary glass mirror. A mirror silvered on the reflecting surface would, I suppose, answer the same purpose; but such mirrors are not permanent, and are troublesome to keep in order while they last. Moreover, if the prism is used only for this purpose, a very small and cheap one will answer, since a pencil half an inch in diameter is all that is required. Such a small right-angled prism is furnished with most large microscopes, and can readily be mounted outside the brass disc so as to answer the special purpose indicated. For all those objects which require the large condensing lens to avoid diffraction and interference, a common glass mirror will answer well enough. For lower powers than two hundred diameters, however, the ordinary mirror will often be found to reflect too much light, and the image on the card-board screen will be found too brilliant to be conveniently observed for any length of time. In such cases a piece of plain unsilvered plate glass may be substituted for the mirror. The greater portion of the solar light passes through it and is lost, but enough is reflected to make pictures of four hundred diameters in from two to three seconds exposure, and these pictures have all the qualities of those made with ordinary mirrors. I have tried instead to diminish the light by absorb ing a part, using for this purpose an ammonio-sulphate cell of considerable thickness, but find that this plan diminishes the contrast and definition of the image, which is not the case when a mirror of simple plate glass is used as above described.

With regard to the management of the plate-holder, the apparatus for focussing, and other accessory arrangements, I need only say that I employ for the solar light the same simple plan which I have described in full in my reports on the use of artificial lights in photo-micrography.

Since making the experiments which have led to the foregoing results, I have modified my method of dealing with the

electric light in photographing the tissues. I first render the divergent pencil proceeding from the carbon points as nearly parallel as possible by means of the condenser, usually supplied with electric lamps for this purpose, and then introduce into the parallel pencil, instead of a ground glass, the very same condensing lens described above for the process with solar light. The image is received primarily on a card-board screen, and the remaining details do not differ from what has been related above. The time of exposure does not exceed a single second for four hundred diameters, and the sharpness of the pictures exceeds any of my former results. Indeed, with this new arrangement, I must say that the electric light appears to me to retain the apparent superiority over sunlight, remarked in my paper on the use of this method of illumination in photo-micrography, at least in the case of all those objects which in themselves possess but little contrast. For well made tissue preparations, however, I find the best work I can do with the electric light so similar to the best attainable by sunlight, used as above described, that I should rarely take the trouble to set up the battery and work the electric lamp, unless it was desirable to work at night or in unfavorable weather.

[The memoir is accompained by photographic plates of tissues, magnified 400 to 500 diameters, as examples of the results obtained in the manner described, and concludes with description of the preparations.]

ART. XXXV.-Barometrical Measurements in Ecuador; by W. REISS and A. STÜBEL. Translated from the Spanish by Professor ORTON, Vassar College.

[THE following is an abstract of the principal altitudes near Quito, as determined by the able North German Expedition in 1870-1. Schmidt's value of the vara, given below, differs from the standard in the U. S. Office of Weights and Measures by -0015 meter. I have reduced the meters to English feet. It is remarkable that every successive measurement of the Quito nian Andes gives a reduced elevation. Thus: altitude of Quito by Humboldt, 9570; Orton, 9520; Reiss and Stübel, 9350;-of Panecillo by Humboldt, 10,244; Orton, 10,101; R. and S., 10,006; of Pichincha by Humboldt, 15,922; Orton, 15,827; R. and S., 15,704;-of the crater by Wisse and Moreno, 13,600; Orton, 13,300; R. and S., 13,175.]

NOTE. The altitudes are calculated in meters above the level of the sea, one meter equaling 1.1963 Spanish vara. The greater part of the observations were made with the barometer; but trig

The

onometrical measurements were taken of some notable points. letters B and T indicate these different means, and the numbers show the times of observations. As this work is provisional, the authors reserve the privilege of making corrections in the future, which, however, will be insignificant.

[merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Junction of Rio Blanco with Rio del Volcan, 2,078

[merged small][ocr errors]

Hacienda of San José in Lloa,

3,091

10,140 26 “

Top of Rucu-Pichincha,

4,737 15,540 T

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

ART. XXXVI.-Inaugural Address before the British Association at Edinburgh, August 2d; by Sir WILLIAM THOMPSON, President of the Association.

*

*

Kew Observatory.

* One of the most valuable services to science which the British Association has performed has been the esablishment, and the twenty-nine years maintenance, of its obervatory. The Royal Meteorological Observatory of Kew vas built originally for a sovereign of England who was a ealous amateur of astronomy. George the Third used contintally to repair to it when any celestial phenomenon of peculiar nterest was to be seen: and a manuscript book still exists lled with observations written into it by his own hand. After he building had been many years unused, it was granted, in he year 1842, by the Commissioners of her Majesty's Woods nd Forests, on application of Sir Edward Sabine, for the purose of continuing observations (from which he had already deuced important results) regarding the vibration of a pendulum

in various gases, and for the purpose of promoting pendulum observations in all parts of the world. The Government granted only the building-no funds for carrying on the work to be done in it. The Royal Society was unable to undertake the maintenance of such an observatory; but, happily for science, the zeal of individual fellows of the Royal Society and members of the British Association gave the initial impulse, supplied the necessary initial funds, and recommended their new institution successfully to the fostering care of the British Association. The work of the Kew Observatory has, from the commencement, been conducted under the direction of a committee of the British Association and annual grants from the funds of the Association have been made toward defraying its expenses up to the present time. To the initial object of pendulum research was added continuous observations of the phenomena of meteorology and terrestrial magnetism, and the construction and verification of thermometers, barometers, and magnetometers designed for accurate measurement. The magnificent services which it has rendered to science are so well known that any statement of them which I could attempt on the present occasion would be superfluous. Their value is due in a great measure to the indefatigable zeal and the great ability of two Scotchmen, both from Edinburgh, who successfully held the office of Superintendent of the Obsertory of the British Association-Mr. Welsh for nine years, until his death in 1859, and Dr. Balfour Stewart from then until the present time. Fruits of their labors are to be found all through our volumes of Reports for these twenty-one years.

The institution now enters on a new stage of its existence. The noble liberality of a private benefactor, one who has labored for its welfare with self-sacrificing devotion unintermittingly from within a few years of its creation, has given it a permanent independence, under the general management of a committee of the Royal Society. Mr. Gassiot's gift of 10.000Z secures the continuance at Kew of the regular operation of the self-recording instruments for observing the phenomena of ter restrial magnetism and meteorology, without the necessity for further support from the British Association.

Physical Observatories and Laboratories.

The success of the Kew Magnetic and Meteorological Obser vatory affords an example of the great gain to be earned for science by the foundation of physical observatories and labora tories for experimental research, to be conducted by qualified persons, whose duties should be, not teaching, but experimenting. Whether we look to the honor of England, as a nation which ought always to be the foremost in promoting physical science, or to those vast economical advantages which must accrue from such establishments, we cannot but feel that experimental re

« ПредыдущаяПродолжить »