that there may be other insects hearing nothing in common with us, but endued with a power of exciting, and a sense that perceives vibrations of the same nature indeed as those which constitute our ordinary sounds, but so remote, that the animals who perceive them may be said to possess another sense, agreeing with our own solely in the medium by which it is excited, and possibly wholly unaffected by those slower vibrations of which we are sensible.

ART. XXVI.-Description of a New Double Image Micrometer for Measuring the Diameter of Minute Celestial Objects. By DAVID BREWSTER, LL. D., F. R. S. Lond. & Ed. &c.

EVERY double image micrometer in which the angle is varied by optical means, must consist of two separate parts, one of which produces the duplication of the image, while the other varies the magnifying power of the telescope, and thus separates the two images, or causes them to approach, till an accurate contact is obtained.

In the micrometer described by the Reverend Dr Pearson, (see this Journal, Vol. III. p. 189,-190.) the double image is produced by a prism of rock-crystal placed between the first eye-glass and the eye, and the variation of the angle is effected by separating the lenses of which it consists. We do not know what part of this instrument Dr Pearson claims as his own; but the variable eye-piece which he uses, was invented by me in 1805; and the prism of rock-crystal is the undoubted invention of that ingenious and amiable man the late Abbé Rochon, who used it both at the object-end of the telescope, in the middle of the telescope, and at the eye-end of the instrument.

The objections to the eye-piece micrometer described by Dr Pearson, may be ranked under three heads:

1. The imperfectly crystallised state of quartz, which is never perfectly homogeneous, and in which minute lines and veins of different refractive powers may be always seen by proper precautions,

2. The difficulty of obtaining the exact sections of the crystal that are required, from the want of natural fractures to guide the artist in his operations.

3. The imperfect achromatism of the eye-piece when the moveable lens or lenses are placed in different positions.

4. The inferiority of achromatic to reflecting telescopes, for examining minute luminous objects with high magnifying powers.

We believe it is now universally admitted, both among astronomers and opticians, that the correction of colour by the best achromatic telescopes, is too imperfect to allow them to be brought into comparison with the fine reflecting telescopes now made in England. For the ordinary purposes of astronomy, the achromatic telescope possesses peculiar advantages; but when it is directed to minute double stars, to luminous points, or to small planetary discs, its performance is inferior to that of the reflecting telescope. For these reasons, a Gregorian or Cassegrainian telescope is employed; and the variation of the magnifying power is produced in the manner which I have described in my Treatise on new Philosophical Instruments, namely, by separating the eye-piece from the great speculum, and procuring an adjustment by a motion of the small speculum *.

Having thus obtained what I conceive to be the most perfect of all methods of obtaining a variation of the angle,—a method, too, in which the performance of the telescope is in no respect injured, we may apply it to the various contrivances which have been invented for giving double images.

In adopting the principle of Rochon as one of the best, I form the doubly refracting prism out of the colourless Topaz of New Holland, which is much freer from veins and imperfections of crystallization than the purest rock-crystal, and has also the advantage of a lower dispersive power. In certain sections of the crystal, when we require only a very small separation of the

It may be proper to mention, that an oversight occurs in the investigation of the nature of the scale of this micrometer, which the mathematical reader will im mediately discover.

images, we may preserve on two sides the natural surface of the cleavage, which I have often found to exceed the finest polish that can be given by art.

When the prism is constructed, it may be made to form part of the lens of the eye-piece next the eye, by cementing it to the outer surface of that lens; or it may be placed between that lens and the eye, as was done by Rochon and Dr Pearson, where it will give a double image of all objects seen through the telescope. The angular distance of these images being invariable for any given position of the eye-piece, they may be brought into contact by a motion of the eye-piece, to or from the great mirror, according as they were previously overlapped or separated.

Although we have proposed to follow Rochon in producing the double image by a doubly refracting crystal, yet there are other means of accomplishing this, which are well worth the attention of the practical astronomer.

1. The double image may be produced by a small bisected plane speculum, placed between the eye-lens and the eye, and one of the halves may be made to move by a screw, not for the purpose of bringing the images in contact, but in order to vary the constant angle, according as it is wanted for large or small discs.

2. The duplication of the image may be effected by bisecting the eye-lens, or by placing a bisected lens between the eye-lens and the eye. We have now before us a bisected achromatic lens, made by Tulley, which produces the two images with very unusual distinctness.

3. The two images may be formed by means of a slightly inclined face, ground upon a highly polished and parallel plate of Fluor-spar; one image being seen by half of the pupil, through the parallel plate, and the other through the inclined face. Fluor-spar is recommended, as producing the least dispersion under a given angle of deviation; but though the colour is perceptible even at small deviations, yet, we are persuaded, from experiment, that it will not occasion any sensible error in the results; and even this imperfection may be removed by the ordi

nary means.

Whatever method of doubling the image the practical astronomer may adopt, after a careful examination of all those that have hitherto been proposed, we beg to press upon his attention the contrivance which we have described for bringing the images in contact. It requires no additional lens or speculum, and gives equally distinct images at every part of the scale.

ABT. XXVII.-Account of the Discoveries of M. OERSTED, respecting the Connection between Magnetism and Galvanism, and the subsequent Researches of Sir HUMPHRY DAVY, Bart. M. AMPERE, and M. BIOT*.

THE connection between Electricity and Magnetism has long been a matter of probable conjecture, not merely from the influence of lightning and artificial electrical discharges on the magnetic needle, but also from the local coincidence between the beams of that decidedly electrical phenomenon, the Aurora Borealis, and the principal magnetic axis of the earth. No decisive experiments, however, were made to establish their identity; and if we except a very curious result, obtained by a member of the Royal Society of Edinburgh, of which we cannot at present obtain the particulars, the sole merit of having effected this discovery is owing to M. Oersted, Secretary to the Royal Society of Copenhagen.

Some years ago an attempt was made by several philosophers, to influence the magnetic needle, by placing it in the open galvanic circuit, but no effect was perceptible; and it occurred to Mr Oersted to make the experiment when the galvanic circle was complete. He immediately found that the magnetic needle

This paper has been drawn up from three articles in the Journal de Physique, tom. xci. p. 72. and other materials, kindly communicated to us by M. Blainville, before they appeared in his own excellent Journal. Sir Humphry Davy's paper has been read at the Royal Society; and we are fortunate in having obtained a correct account of its leading results.-D. B.

was moved from its position; but as his apparatus was feeble, and the results not strongly marked, he associated himself with his friend M. Esmark, Counsellor to the King, and provided a galvanic apparatus, consisting of twenty copper troughs, each of which was 12 inches square, with a breadth of about 2 inches. Each trough was furnished with two plates of copper, disposed so as to support the rod of copper which sustains the zinc plate in the fluid of the next trough. The conducting fluid consisted of pure water, containing th of its weight of sulphuric acid, and as much nitric acid. The portion of each zinc plate immersed in the fluid is a square whose side is about ten inches long. A less powerful battery will be sufficient, provided that it is able to make a metallic wire red-hot. The opposite extremities of the pile are joined by a metallic wire, called the Uniting Wire (fil conjonctif, by Ampere and Biot,) and the name of the Electric Conflict was given to the effect which took place in the uniting wire, and in the space around it.

1. Above a magnetic needle, well suspended, and in equilibrio in the magnetic meridian, is placed a straight part of the uniting wire, so as to be horizontal and parallel to the needle, which may be done by bending it near its efficacious part. When this is done, the needle will be found to deviate from its position;-the pole which is nearest the negative end of the battery will move to the westward, and if the distance of the needle from the uniting wire does not exceed three-fourths of an inch, the declination of the needle will be 45. At greater distances the declination decreases proportionally; and the distance remaining the same, the declination varies with the strength of the battery.

If the uniting wire is placed below, instead of above, the needle, the effects will be inverted, and the pole which is nearest the negative end of the battery will move to the Eastward.

2. Hence it follows, in general, that if NEGATIVE electricity enters ABOVE the pole of the needle, it will decline to the WEST, and if it enters BELOW, it will decline to the EAST.

If the uniting wire is made to turn in a horizontal plane, so as to deviate gradually from the magnetic meridian on either side, the declination of the needle will increase, if the wire approaches the needle, and will diminish if it recedes from it.