may be, an experienced eye will always be able to see the uncorrected achromatic aberration by this test. Small holes drilled in a thin metallic plate and blacked, form a similar object, and generally all dark bodies seen against intercepted light show the same phenomena.

A very valuable object to the working optician is an artificial star; this may be made by a very minute glass globule stuck on a black ground, or by some grains of platina fused by electricity. Mr. Lister first employed globules of quicksilver for the same purpose.

His way of procuring them is, I believe, as follows:

Let a small quantity of mercury be squeezed through leather, to clean it; then put it into a glass tube with some water having a very little gum dissolved in it; agitate it till it resolves itself into very minute globules; extract a few of them, and let them be secured on some blacked paper with the help of a little thin gum-water; they may then be crushed with the finger so as to be totally invisible to the naked eye: the smaller they are the better.

The light of the sun falling upon them produces a beautiful disk, but common day or candle light does pretty well also. The initiated well know how to appreciate the use and value of this object, which is, perhaps, the best which can be employed to verify the state of the figure, achromatism, centreing, grinding, and adjustment, &c. of the small aplanatics. All their defects may be clearly looked into and probed by the means of this single object, and the proper remedies applied to them, which, of course, cannot be done unless the nature of the disease is duly ascertained.

Fig. 12 is the foot of a blue-bottle fly, an interesting opaque object, and an admirable test, its minutiæ requiring a great deal of good distinct light, combined with considerable power to become duly manifest. The specimen I have drawn is, I believe, one of the hinder feet, for the three pair all differ somewhat from each other in the proportion of their component parts, though alike in their general structure: (as one of the claws was buried in the gum which was used to stick the foot on the black surface to which it was applied, I have not represented it.) I lament moreover that my draw

ing must be considered as having been taken from a dried specimen instead of a recent one, (as it should have been,) for it is an object which is sure to be much changed, warped, and shrivelled up, if detached from a newly-killed fly, even for one hour only. However, as the prominent features which constitute the peculiar subjects of examination as tests were well preserved, the drawing will do well enough to point out what I wish to submit to my readers' consideration.

At a is to be seen a fluting in the claw; b, b, b, b, b, are lines, furrows, or flutings in the hairs of a whitish tint; c is a species of scale-work on the shank somewhat similar to what may be observed on the feet and legs of birds; d d are minute white dots, probably the orifices of glands, dispersed over the whole of the inferior surface of the suckers as represented in the plate; e is a fringe of hairs round the margin of the suckers, with minute globules at their terminations.

Now all these minutiæ can be seen, in more or less perfection, in every good specimen. I must observe, however, of the dots or granulations on the soles of the feet, that they are sometimes not visible, owing to the whole surface being occasionally furred over with dust, dirt, and other extraneous matter, which is apparently licked up by the glutinous fluid which exudes from the orifices of the glands. I think that kind of light which is afforded by a silver cup does best upon

the whole for this object, and is, indeed, absolutely necessary to give the Amician reflector the power of showing this object advantageously, though I have managed to see every thing there is to be seen about a foot, without its assistance to the effect of the said instrument. I may remark, that, being a very sombre object, it is peculiarly calculated to be seen unfavourably in the dark image of the reflector; nevertheless, every point can be shown by the Amician metals of 0·6 focus and 0.3 aperture completely well, and with as much ease too as by any other instruments greatly superior to them in mere light or brilliancy of effect. A dark ground of some kind or other is absolutely necessary to exhibit this object in its proper relief; a white one confuses it altogether; the power may vary from to inch : candle or lamp light, I think, does best for it, and the darker the apartment is made in which we observe it the better, for darkness causes the iris of the eye to expand, and receive a stronger impression from the faint rays issuing from such dull objects.

A human hair is a beautiful proof object; I repent that I have not given a figure of it.

Mr. W. Tulley has discovered a singular property in almost all hairs, which I do not think has ever been remarked or described. If a hair is drawn between the finger and thumb from the end to the root, it will be distinctly felt to give a greater resistance and a different sensation to what is experienced when drawn the opposite way; in consequence, if the hair is rubbed between the fingers it will only move one way, (travelling in the direction of a line drawn from its termination to its origin from the head or body,) so that each extremity may thus be easily distinguished even in the dark by the touch alone.

The mystery is resolved by the microscope. A hair viewed on a dark ground as an opaque object, with a high power not less than that of a lens of of an inch focus, and dully illuminated by a cup, which seems to answer best, is seen to be indented with teeth somewhat resembling those of a coarse round rasp, but extremely irregular and rugged; as these incline all in one direction, like those of a common file, viz. from the origin of the hair towards its extremity, it suffi

ciently explains the reason of that singular property I have described.

This is a singular proof of the acuteness of the sense of feeling, for the said teeth may be felt much more easily than they can be seen. We may thus understand why a razor will cut a hair in two much more easily when drawn against its teeth, than in the opposite direction.

P. S. The instrument which I have in this paper termed the Amician Microscope is not the legitimate offspring of Amici, (from whom its principle only has been borrowed); but an improvement of mine, executed by T. Cuthbert. The profound critical acumen and superior taste of the professor (as I am given to understand) wholly rejects and disowns the particular modification in question, as a spurious and bastard variety, which must be affiliated upon myself, being wholly different from his in every thing but the optical principle. With this decision I am perfectly content.

ASTRONOMICAL AND NAUTICAL COLLECTIONS. July, 1827.

i. A Letter from Mr. HENDERSON, containing a corrected Method of computing an observed OCCULTATION.

DEAR SIR,

Edinburgh, 3, Leopold Place, May, 1827.

I THINK that my Rules for computing an observed Occultation may be improved; and, in order to enable you to judge of this matter, I beg leave to inclose a copy of the corrected rules with an example. I give the preference to the corrected rules for the following reasons:

1. When the star is near the zenith, a considerable degree of uncertainty attends the altitude computed by the method in the Requisite Tables, on account of the small variations in the sines of arcs approaching to 90°: and this uncertainty exercises a considerable influence upon the parallactic angle

and parallaxes, as I have several times remarked in cases which have actually occurred.

2. The affection of the parallactic angle is sometimes ambiguous, as noticed in Remark 1.

But, in the corrected rules, these imperfections are avoided by computing the parallactic angle by Napier's analogies. 3. By the rules in the N. A. the parallactic angle is directed to be reckoned sometimes from the North, and sometimes from the South Pole. But by the corrected rules, this angle is always reckoned from the North Pole, whereby the precepts are simplified.

Other changes have been made; and, if you think proper, the corrected rules may be published in such manner as you deem suitable. And I am, with respect,

may

Dear Sir,

Your very obedient humble servant.

THOMAS HENDERSON.

Rules for computing an observed Occultation.

1. FIND half the sum and half the difference of the north polar distance of the zenith (corrected on account of the spheroidal figure of the earth), and the north polar distance of the star; and to the logarithmic cosecant of the half sum add the log. sine of the half difference, and the log. cotangent of half the star's horary angle; the sum is the log. tangent of an angle A always less than 90°: and to the log. secant of the half sum add the log. cosine of the half difference, and the log. cotangent of half the horary angle, the sum is the log. tangent of an angle B, which is greater or less than 90°, accordingly as the half sum is greater or less than 90°. If the north polar distance of the zenith be greater than the north polar distance of the star, the sum of A and B, otherwise their difference, is an angle C, to which prefix the sign + when the star is east of the meridian, and when west. When the orbital angle has N. prefixed, its complement to 90° will have the sign-, and when S. The sum of this com