Example. If the limiting aperture of the 9th mag. be 1 inch, what aperture will gauge a 3rd mag. star? (2.512) 3 = 063 Generally, if the limiting aperture for the 9th mag. be 1 inch, n = 9+ 5 log. where log. 9 will be negative if be less than 1; and from which the magnitude of a star can be immediately found as soon as its limiting aperture has been determined by observation. Table of the relative Amounts of Light received from Stars of the different Magnitudes, and the corresponding Limiting Table of Magnitudes corresponding to Limiting Apertures. I may as well mention here a very simple arrangement of mine for measuring the limiting apertures of stars. A cap is made to fit the object end of the telescope, carrying two sliding brass plates, the under one of which has the largest possible equilateral triangle cut out of it that the object-glass can completely cover. The upper plate slides smoothly over the triangle parallel to one of the sides, so that whatever aperture be open it will be an exact equilateral triangle. A scale graduated to 50ths of an inch, and perpendicular to the motion of the upper plate, indicates the height of the open triangle, and the diameter of the circular aperture of equal area is found from the expression, d = [9*93317] h This cap is made at small cost by Cooke, of York, and is a capital photometer. I fear I have trespassed somewhat on your space, but it's only once Yours very obediently, in a way. Fort Cumberland: 30th August 1865. G. L. TUPMAN. THE WILLOW LEAVES ON THE SUN. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In your last impression, I notice a letter from Mr. Nasmyth, taking exception to a remark that I made at the Royal Astronomical Society, to the effect that "I considered the willow leaves of Mr. Nasmyth were not the same as the rice grains seen on the sun's surface." As many of your readers may not have the opportunity of seeing the Monthly Notices of the Royal Astronomical Society, and my paper therein upon the Solar Photosphere, which gives my reason for considering these expressions as referring to different bodies, I have therefore written this letter to show whether I have not a good plea for my opinion. I believe that most persons having telescopes of 3-in. aperture and upwards, have been able to see without difficulty the so-called rice grain forms; I have estimated the size of the largest of these seen by me at about 8′′ of arc long, and about 2′′ of arc broad +. Now, the willow leaves I have never seen, so I must take Mr. Nasmyth's own authority for their size (see Monthly Notices R. A. S. vol. xxiv. p. 66)— to wit, 1000 miles long, and about 100 miles wide, or 2" of arc long, and "of arc wide +. Now, this size is thirty times smaller than the rice grains that I can see; surely I have good reason for considering that the willow leaves and the rice grains are different formations. My estimate of size is too much for an average, I believe; still, the disparity is too great for coincidence. I do not believe it possible to define an object of the size quoted by Mr. Nasmyth, on the sun's surface, in any telescope of 3-in. or 4-in. aperture. I should find considerable difficulty myself in my own equatoreal telescope, which is of 8-in. aperture. The scrutiny of the sun's surface is no easy matter. Father Secchi, writing from Rome in January 1865, says (in reference to willow leaves): "If by this one means the small little bits, or commas, delineated in the figure which constitutes the frontispiece The plate referred to was taken from a drawing made by Mr. Nasmyth for the work in question. of the Handbook of Astronomy of Mr. Chambers, I must confess that I do not see them." Again, "The name of leaf supposes a certain regularity and uniformity, which has not been seen by me." And again, "I cannot divest myself of the opinion that the photosphere is really made up of clouds, and that the luminous stratum is actually constituted like our clouds, the only difference being that the clouds on the earth are of watery drops or crystals, and in the sun they are of some other substance," &c. My own observations regarding these clouds coincide entirely with Father Secchi, which in effect is this: That the whole of the sun's photosphere is composed of a cloudy stratum, formed of masses of matter, remarkably similar in shape to the cumuli clouds of our earth, varying, however, very frequently round the edge of the penumbræ of solar spots, where it assumes a shape similar to that of straws, or thatching, or willow leaf, or any such term denoting a filamentous structure, evidently showing the existence of some current of atmosphere, or gas, passing over the edge of the penumbra down to the bottom of the sun spot, or the umbra, and so drawing out the exceedingly mobile photosphere into filamentous forms, such as is constantly seen in this country, where a current of air passes over a range of hills, on the top of which a fog-cloud hangs. That the sun's surface is composed of matter shaping itself in nodules, such as our clouds constantly assume, there can be no doubt; or as Sir John Herschel has so aptly described "as a flocculent chemical deposit;" when definition has been very good, these nodules have appeared so sharply defined, that I have compared them to a coarse shingle beach in appearance, where stones of all diversities of rounded and oval shapes are prevalent-hence the term rice grain; but that the surface is composed of the interlacing of entities such as willow leaves, is "the great fact" yet to be proved, before some of our best observers will be convinced of its truth-for as yet they are still incredulous, though possessing instruments as large and even larger than those of Mr. Nasmyth. I beg to remain, Sir, yours obediently, Molesey Gore, Uckfield, Sussex: FREDERICK BRODIE. LUNAR OBSERVATIONS. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-As it appears to be generally admitted that, under the present system of observation, a period of many years will be probably required for the completion of the new lunar map, and in addition to the loss of time therefrom resulting, which is much to be regretted, a greater difficulty seems likely to arise-viz. a considerable difference of epoch-permit me to suggest that the principle of division of labour might here be applied with advantage. Suppose the whole visible surface of the moon divided into (e.g. say) 900 parts, and dis These nodules are generally nearly round, and vary from 1" to 2" in diameter; when much elongated in form, they become rice grains, and sometimes attain to 8" in length. tributed, one or more parts to each, among a number of astronomical observers possessed of telescopes of sufficient power, and such standing as would enable perfect reliance to be placed upon their observations since the portion allotted to each observer would amount only to 1' or 2', I trust there would be little difficulty in finding a sufficient number of volunteers for the work, while the whole attention of the observer being thus concentrated on a very small area during at least two lunations, the details could be examined and the exact appearance of each feature of our satellite's surface studied with the utmost precision. Of course it would be necessary to furnish each observer with printed forms, or proportions, that each portion might be represented on the same scale as the rest, so as to admit of their easy combination into a single map. If to the final production an index were furnished, showing to what observer the responsibility of each delineation belonged, a selenographic map of the greatest accuracy might be easily prepared by this method in a few months, and perfect identity of epoch attained. Commending this suggestion to the consideration of the lunar committee, and your readers generally, I remain, Sir, yours faithfully, SIDNEY B. KINCAID. MARE CRISIUM. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Dear Sir,-May I trouble you once more with some observations on the locality of Picard? On Sept. 5, I was looking at the region near Picard, and, to my astonishment, noticed a conspicuous bright spot s of the spot I c2, and then almost equal in light. On the last lunation I noticed, in the immediate locality of this spot, a most minute crater (when near the terminator) very difficult to make out. Last night the terminator was again near the crater, when it appeared most conspicuous-quite as much so as I cu2. I took some rough measurements, which gave for its angular position, reckoning from the line joining the N. point of Picard to the N. of d of B and M, 148°, and the distance from the N. wall of Picard was 16.5 ±. The misty cloud, which I estimated at 3", surrounding this little crater, I think, should be very carefully watched and measured. We have had one instance of a cloud fading away, and this may be one appearing. Any bright spot seen under high illumination should, if possible, be identified so that it may be seen whether it is on or near a crater. This region seems a likely one for change, and should be well looked after whenever practicable. I should be glad, if possible, for you to insert this in the Register for October, that lunar observers may take notice in time, And remain yours very faithfully, HERBERT INGALL. P.S.-Telescope 4-in. aperture, power 300. Camberwell: Sept. 8, 1865. JUPITER'S SATELLITES: BRIGHT SPOTS ON THE MOON. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Allow me to thank you for your reply to my enquiries respecting Jupiter's satellites. The existence of a penumbra surrounding their shadows when in transit is very interesting, as showing that in this respect, at least, this noble planet resembles our own earth. As to "bright spots on the moon," your correspondent "Juvenis " is in error if he supposes my object was identification with a known object. As further details of it may be interesting, I send the following particulars: 1865. January 1st, 6 P.M. The sky being very clear, on directing my telescope to the moon, I was struck with the clearness with which its unenlightened part was seen. My attention was immediately attracted by a small speck of light, which I carefully studied, and noted the following particulars :— Position: This I paid great attention to. I could see the outlines of the Mare Imbrium, Plato, and other large objects, and I carefully estimated the speck as situated close under the east foot of the Alps, very near the mouth of the wedge-shaped valley. Appearance: A star of about 4th magnitude, a very little out of focus, a little light haze surrounding it, instantly reminding me of Herschel's idea of a glowing coal covered with ashes. Applied powers 50, 65, 80; best seen with 50. Time of observation: I kept the speck in view for fully half-anhour, during which it underwent no change, neither did it twinkle like a star; its light was quite steady, and it seemed rather larger than a star of equal brightness would do. These are the particulars of this object, and, although I have carefully re-observed the place many times since, I could never see it, neither can I identify it with any known object. I am, Sir, yours very truly, Red Lion Street, Chesham, Bucks: September 10, 1865. CHARLES GROVER. NOTES AND GLEANINGS. NEW MINOR PLANET.-M. Luther, at Bilk, on the night of August 25, discovered a new planet in the following position :— This new accession to the list of planetoids will bear the number 84. Up to the time we write, we have not heard of its being named. |