magnetic force passing through the metals is the same in all the cases, and hence the polarity within them the same.

The author then gives a more explicit meaning, in accordance with the definition of line of magnetic force contained in this paper, to some of the expressions used in the three last series of his Researches on Magnetic Condition, Atmospheric Magnetism, &c. : and by referring to former results obtained since the year 1830, illustrates how much the idea of lines of force has influenced the course of his investigations, and the results obtained at different times, and the extent to which he has been indebted to it; and then, recommending for many special reasons the mode of examining magnetic forces by the aid of a moving conductor, he brings for the present his subject to a conclusion.

ROYAL ASTRONOMICAL SOCIETY.

[Continued from vol. ii. p. 326.]

June 13, 1851.-On some Improvements in Reflecting Instruments. By Prof. Piazzi Smyth.

In the course of his lectures on Practical Astronomy to the students of the Edinburgh University last winter, Prof. P. Smyth had unusual opportunity of ascertaining those points in the making of the generality of the observations of navigators by sea and of travellers on land, which presented the greatest difficulty to beginners. And as these points generally consisted of needless peculiarities, sometimes absolute imperfections in the instruments, the Professor proceeded to remove them as well as he could, and the result may, perhaps, be more extensively useful, especially as the difficulties were generally felt on the sextant being applied to observations of stars by night, a more exact means than the sun by day, and therefore to be encouraged and assisted in every way. In naval observations the impediments were, both by the experience of the class, and by the testimony of naval officers,— 1st. Difficulty of seeing and of bringing down the star. 2nd. Difficulty of seeing the horizon line at night. 3rd. Difficulty of reading off the angle on the limb.

The first of these, in so far as it depended on the dark field of the telescope, he proposed to remedy by employing a telescope of large aperture, say 2 inches, in place of the usual size, or inch; in so far as the loss of light was occasioned by reflexion and absorption at the glasses, he intended to remove this by employiug metal reflectors, by which, too, the occasional nuisance of second images would be avoided and greater accuracy obtained. He had tried speculum metal for the purpose with great advantage; but, under some circumstances, he was in hopes of being able to employ silver, which has lately been found to be capable of reflecting near double the amount of light that speculum metal does, though that retains more than quicksilvered glass; and then, in so far as the loss of the star in "bringing down" is caused by the diminished surface exposed by the index-glass at large angles, he proposed to make that larger than usual: besides which, the reflexion taking

place in the metal at the first surface, there would be no loss, as now, from the thickness of the edges of the glass or the sides of the brass box containing it.

The second difficulty would be alleviated by the same adoption of the large object-glass: besides the loss of light by transmission through the so-called transparent part of the present horizon-glass would be done away with by the employment of the metal reflectors. The third difficulty was also shown to be gratuitous, for the reflector of the reading-glass was in general so placed that the light of the lamp could not get to it, and if it did, would be thrown away from the arc instead of on it: and were even that managed, the surface of the vernier and arc being in different planes, the same ray of light would not illumine them both at the same time. By placing them, however, both in the same plane, and by putting the reflector at an angle of 45° to the limb, instead of parallel to it, so as to receive parallel light and throw it straight down to the divisions, it was found that they could easily be read by a very faint light.

For accuracy, opposite readings were deemed essential, and a circle insisted on in place of a sextant or quadrant: and the author, considering that the failure of the reflecting circle in securing a permanent footing in the navy arose from its being made in general too large and heavy, and complicated, he had devised a very small, but strong and simple form; the telescope was more firmly connected by moving in grooves on the large surface of the face of the circle, instead of rising by the usual single screw; and in place of the inconvenient plan of having to reverse the hands so as to put the instrument into its box face uppermost (which makes the getting of it out again without pulling at the reflector or some such delicate parts, difficult), by placing the legs not on the back but on the face, the instrument may be either put into its box, or down on the floor, or anywhere, face first, with the same hand which was moving it in the observation, with the divisions and the reflectors protected from all accidents, and the whole instrument ready at any time on a moment's notice (for the telescope never need be taken off, with its improved fixing), to take advantage of an instantaneous opportunity of observation.

So much for the use of the reflecting instrument at sea: as used on land, the following difficulties were found, and are generally recognised :

1st. The impossibility of measuring in the mercury either sun or star when within, say 20° of the zenith, from the reflecting instrument not taking in so large an angle; and again, when within, say 10° of the horizon, from the foreshortening of the reflecting surface.

2nd. The difficulty of seeing the referring point all night, viz. the reflected image of the star, when black glass is employed; and the trouble with wind when employing mercury, as well as with other causes producing vibration; and the great weight and liability to loss in long journeys through difficult and uncivilized countries.

All these difficulties seemed to be met by making the reflecting

surface of speculum metal, leveled by a spirit-level; and when the reflected object could not be seen, attaching to the metal a collimating telescope, whose optical axis was parallel with the previously leveled surface, and was defined at the focus end by a horizontal slit, illuminated by a lamp at night, so as completely to remove all difficulty of seeing the referring object, and allowing of almost the whole object-glass being brought to bear on the star.

Difficulty having been found by the students in keeping sight of an object reflected from the artificial horizon, the latter was generally placed on a stand so as to bring it near the eye, and make it thereby offer a large angular space, which was pretty sure not to be exceeded by the shaking of the hand or involuntary movement of the head of an unpractised observer; but it was found requisite, not only to make the stand firm, but to improve the steadiness of the leveling screws, which was done by making them parts of a fixed frame, with the reflector moveable on them, and capable of being fastened in any position between opposite nuts.

A sextant with all the improvements (except the opposite readings), a full-sized model of a circle, and one of the reflecting horizon, were shown; but Prof. Smyth did not mean to claim any part of them as his own invention; for without making any special inquiries as to how far he might have been preceded by any one else, he believed that he had only brought to bear on this subject individual improvements long and well known in other departments of the science; but as they had never, he thought, been so completely united before, and as such a reunion might enable observations often to be obtained when now they are given up, he hoped that the communication might not be uninteresting to some of the numerous working members of the Society,

XII. Intelligence and Miscellaneous Articles.

ON THE PRODUCTION OF INSTANTANEOUS PHOTOGRAPHIC IMAGES. BY H. F. TALBOT, ESQ.

IT. T will probably be in the recollection of some of your readers that in the month of June last a successful experiment was tried at the Royal Institution, in which the photographic image was obtained of a printed paper fastened upon a wheel, the wheel being made to revolve as rapidly as possible during the operation.

From this experiment the conclusion is inevitable, that it is in our power to obtain the pictures of all moving objects, no matter in how rapid motion they may be, provided we have the means of sufficiently illuminating them with a sudden electric flash. But here we stand in need of the kind assistance of scientific men who may be acquainted with methods of producing electric discharges more powerful than those in ordinary use. What is required, is, vividly to light up a whole apartment with the discharge of a battery :-the photographic art will then do the rest, and depict whatever may be moving across the field of view.

I had intended to communicate much earlier the details of this experiment at the Royal Institution, but was prevented from doing so at the time; and soon afterwards I went on the Continent in order to observe the total solar eclipse of the 28th of July. This most interesting phænomenon I had the pleasure of witnessing at the little town of Marienburg, in the north-eastern corner of Prussia. The observations will appear, I believe, in a forthcoming volume of the Transactions of the Royal Astronomical Society. Among other things, I was enabled to make a satisfactory estimate of the degree of darkness during the total obscuration; which proved to be equal to that which existed one hour after sunset the same evening, the weather being during that evening peculiarly serene, so as to allow of a just comparison.

This Continental journey having effectually interrupted my photographic labours, I have only recently been able to resume them. I shall therefore now proceed to describe to you exactly the mode in which the plates were prepared which we used at the Royal Institution; at the same time not doubting that much greater sensibility will be attained by the efforts of the many ingenious persons who are now cultivating the art of photography. And it is evident that an increased sensibility would be as useful as an augmentation in the intensity of the electric discharge.

The mode of preparing the plates was as follows :—

1. Take the most liquid portion of the white of an egg, rejecting the rest. Mix it with an equal quantity of water. Spread it very evenly upon a plate of glass, and dry it at the fire. A strong heat may be used without injuring the plate. The film of dried albumen ought to be uniform and nearly invisible.

2. To an aqueous solution of nitrate of silver add a considerable quantity of alcohol, so that an ounce of the mixture may contain three grains of the nitrate. I have tried various proportions, from one to six grains, but perhaps three grains answer best. More experiments are here required, since the results are much influenced by this part of the process.

3. Dip the plate into this solution, and then let it dry spontaneously. Faint prismatic colours will then be seen upon the plate. It is important to remark, that the nitrate of silver appears to form a true chemical combination with the albumen, rendering it much harder, and insoluble in liquids which dissolved it previously.

4. Wash with distilled water to remove any superfluous portions of the nitrate of silver. Then give the plate a second coating of albumen similar to the first; but in drying it avoid heating it too much, which would cause a commencement of decomposition of the silver. I have endeavoured to dispense with this operation No. 4, as it is not so easy to give a perfectly uniform coating of albumen as in No. 1. But the inferiority of the results obtained without it induces me for the present to consider it as necessary.

5. To an aqueous solution of protiodide of iron add first an equal volume of acetic acid, and then ten volumes of alcohol. Allow the mixture to repose two or three days. At the end of that time it

will have changed colour, and the odour of acetic acid as well as that of alcohol will have disappeared, and the liquid will have acquired a peculiar but agreeable vinous odour. It is in this state that I prefer to employ it.

6. Into the iodide thus prepared and modified the plate is dipped for a few seconds. All these operations may be performed by moderate daylight, avoiding however the direct solar rays.

7. A solution is made of nitrate of silver, containing about 70 grains to one ounce of water. To three parts of this add two of acetic acid. Then if the prepared plate is rapidly dipped once or twice into this solution it acquires a very great degree of sensibility, and it ought then to be placed in the camera without much delay.

8. The plate is withdrawn from the camera, and in order to bring out the image it is dipped into a solution of protosulphate of iron, containing one part of the saturated solution diluted with two or three parts of water. The image appears very rapidly.

9. Having washed the plate with water, it is now placed in a solution of hyposulphite of soda, which in about a minute causes the image to brighten up exceedingly by removing a kind of veil which previously covered it.

10. The plate is then washed with distilled water, and the process is terminated. In order, however, to guard against future accidents, it is well to give the picture another coating of albumen or of varnish. These operations may appear long in the description, but they are rapidly enough executed after a little practice.

In the process which I have now described, I trust that I have effected a harmonious combination of several previously ascertained and valuable facts-especially of the photographic property of iodide of iron, which was discovered by Dr. Woods of Parsonstown, in Ireland, and that of sulphate of iron, for which science is indebted to the researches of Mr. Robert Hunt. In the true adjustment of the proportions, and in the mode of operation, lies the difficulty of these investigations; since it is possible by adopting other proportions and manipulations not very greatly differing from the above, and which a careless reader might consider to be the same, not only to fail in obtaining the highly exalted sensibility which is desirable in this process, but actually to obtain scarcely any photographic result at all.

To return, however, from this digression.-The pictures obtained by the above-described process are negative by transmitted light and positive by reflected light. When I first remarked this, I thought it would be desirable to give these pictures a distinctive name, and I proposed that of Amphitype, as expressive of their double nature— at once positive and negative. Since the time when I first observed them, the Collodion process has become known, which produces pictures having almost the same peculiarity. In a scientific classification of photographic methods, these ought therefore to be ranked together as species of the same genus. These Amphitype pictures differ from the nearly related Collodion ones in an important circumstance, viz. the great hardness of the film and the firm fixation of the image, which is such that in the last washing, No. 10, the image