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which, when the incident rays are parallel, must be taken for the secondary caustic in the place of the Cartesian, which, for the particular case in question, passes off to infinity. In the course of the memoir, the author reproduces a theorem first given, he believes, by himself in the Philosophical Magazine, viz. that there are six different systems of a radiant point and refracting circle which give rise to identically the same caustic. The memoir is divided into sections, each of which is to a considerable extent intelligible by itself, and the subject of each section is for the most part explained by the introductory paragraph or paragraphs.

By Dr.

GEOLOGICAL SOCIETY.

[Continued from vol. xii. p. 482.] November 19, 1856.- Col. Portlock, President, in the Chair. The following communications were read :

1. “On the Occurrence of crystallization in Stucco.” Buist. Communicated by Sir R. I. Murchison, F.G.S.

The author remarked that in the spring of 1848 a quantity of fragments of plaster-castings which he had thrown out of doors were, after exposure to the atmosphere and rain, before long covered with crystals of selenite. Specimens, communicated by Dr. Buist, were exhibited, together witli calcareous casts of shells and tubular bodies, from the upraised beaches of the coast of India, which the author sent and described as exhibiting other interesting instances of alteration in molecular arrangement.

2. “On the Occurrence of Allophane at Charlton, Kent." By J. Morris, Esq., F.G.S.

The author described the circumstances under which allophane occurs in the fissures of the Chalk near Woolwich, and pointed out that its formation was evidently of later date than the disturbances of the Chalk and the overlying Thanet sands, and of the determination of the present physical features of the district.

An analysis made by Mr. Dick gave the following composition:

Silica 18:89, alumina 33.52, water 42:73, lime 1.67, carbonic acid 2:51, organic matter, a trace.

Mr. Morris suggested that this allophane was probably due to the percolation of atmospheric waters through the superficial deposits, and the sands immediately overlying the chalk; and he pointed out that probably Scarbroite (or Kollyrite), Pholerite, and several other allied hydrosilicates of alumina should not be regarded as distinct mineral species; the slight variation in their chemical formulæ having been probably determined by the local conditions under which each of them originated.

3. “On the Red Sandstones and Quartzites of the North-west of Scotland.” By Prof. James Nicol, F.G.S.

The district described in this communication stretches, N.N.E. and S.S.W., from Cape Wrath and Durness to Sleat, the southern portion of the Isle of Skye, with an extent of more than 100 miles and a breadth of from 15 to 30 miles. Its structure was illustrated by transverse sections afforded by Loch Broom, across the centre of

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the district, by Loch Assynt, the Kyles of Durness, and Loch Eriboll, on the north, and on the south by Lochs Greinord, Gairloch, Maree, Keeshorn, and Carron, and by a section of the southern part of Skye. These sections present more or less uniformly the following ascending series of rocks, which, excepting the lower gneiss, have in general an inclination towards the S.E.:-1. Gneiss, with some granitic veins, and having a general N.W. and S.E. direction. 2. Red conglomerates, grits, and sandstone, lying unconformably on the gneiss, and exposed along a tract of about 20 miles in width: these beds are at least 3000 feet thick, but rapidly thin off eastward. 3. White quartzite, unconformable to the red sandstone, and frequently accompanied by an overlying, hard, dark-coloured, siliceous limestone; these form a band about 10 miles wide, and are in places at least 500 ft. thick. 4. Gneiss, occasionally of a different character from the lower gneiss, and distinctly seen at many localities to overlie the quartzite. Gneiss also constitutes the country to the eastward.

These rocks, with occasional serpentines, porphyries, and other igneous rocks, were described in detail ; and it was stated that the red sandstones (No. 2) were apparently unfossiliferous; that the quartzite presented some obscure evidences of organic remains; and that the limestone at Durness had furnished some univalve shells, such as Euomphalus and Orthoceratites, besides other indistinct fossils, first observed by Mr. Peach.

The author pointed out that the red sandstones and conglomerates (No. 2), occurring along the district described, decidedly form parts of one series, and that, in his opinion, they are referable to the Devonian age, whilst the overlying and unconformable quartzite and limestone (No. 3), judging from their position and mineral character, represent the Lower Carboniferous series of the South of Scotland; although in this he necessarily differs from Sir R. I. Murchison's opinion, that the beds in question represent a part of the Silurian series.

Prof. J. Nicol alluded to the difficulty of accounting for the superincumbent position of the gneiss, No. 4, which overlies the abovedescribed sedimentary rocks on the west, whilst a similar rock underlies the true Old Red formation on the east, in Caithness. Assuming, however, the continuity of the great mass of gneiss, he points to certain igneous rocks, and other local evidences of disturbance along the eastern limit of the quartzite, which may have resulted in the forcing up of a portion of the lower gneiss over the sedimentary beds; whereas, on the other hand, the great degree of alteration visible in the upper limestone and quartzite rather indicates, in his opinion, that a metamorphosis in situ has affected the uppermost beds of the series.

The author concluded with a sketch of the physical changes which this district must have successively undergone, from the deposition of the conglomerates to the period of the elevation of these Highlands into the region of ice and snow, and of the formation of enormous glaciers among their ravines, traces of which still remain in the “perched blocks" and local accumulations of drift, and in the highly-polished surfaces of their hardest rocks. This period was followed by subsidence, and by a still later partial elevation.

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XII. Intelligence and Miscellaneous Articles.

THE TELESCOPIC STEREOSCOPE.
To the Editors of the Philosophical Magazine and Journal.

GENTLEMEN,
PERHAPS you will allow me a corner in your next Number to

say that I have recently succeeded in constructing what I believe to be a new form of the stereoscope. Its object is to unite large binocular photographic pictures in a different way from any that has hitherto been followed.

The pictures are placed side by side, and viewed through two small telescopes, like those of opera-glasses, with the directions of their axes crossing each other; the left-hand picture being viewed with the right eye, and the right-hand picture with the left eye. The two telescopes are connected together, the connecting apparatus being capable of two adjustments; one to suit the width of the eyes, and the other to give the obliquity required. When the instrument is placed on a stand, as I have it, two other adjustments are required; the first to bring the telescopes to the proper elevation, and the second to bring the plane of their axes into parallelism with the upper or lower margins of the pictures.

The instrument is constructed in such a way that these adjustments are made with great facility; and when the pictures are united, the effect is excellent.

If you have any reason to suppose that the instrument is not new, I will thank you to commit this to the flames; but if otherwise, I will trouble you with a more comlpete description in your succeeding Number:

In the mean time, on the supposition that I have the priority in this instance, I propose to call the instrument the Telescopic Stereoscope.

I need scarcely say that the pictures to be viewed are transposed from their usual position when viewed in the lenticular stereoscope, that which usually occupies the right side being placed on the left.

Of course the telescopes must be of the same magnifying power, and in every respect alike.

Gentlemen, 1 St. Vincent Street, Edinburgh,

Your obedient Servant,
December 12, 1856.

James Elliot.
ON THE COLORATION OF THE MOON DURING ECLIPSES.

BY M. FAYE, It is well known that the portion of the lunar disc which is immersed in the cone of the shadow of the earth still receives rays refracted by our atmosphere, and that it then appears tinted with a copper-red, or rather brown, the shade of which is rather difficult to define.

The last eclipse has given me the opportunity of ascertaining that this tint not real, or at least that it is greatly altered by an effect of that simultaneous contrast of which Chevreul has made known the laws, and of which he has lately brought an interesting example before the Academy. It was sufficient for me to cover the non

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eclipsed portion of the moon by a distant obstacle, such as the projection of a roof or the cornice of a chimney, to see the colour of the eclipsed portion change its tint completely; instead of a brownishred, I only saw a bright rose-colour, identical with that which so often tints elevated clouds at the rising or setting of the sun. The peculiar colour which sullies this beautiful tint when we look at the entire moon in an incomplete eclipse, is therefore an effect of contrast due to the yellowish tint of the ordinary light of our satellite.

As to total eclipses, if the reddish-brown tint persists notwithstanding the want of all contrast, it is because an effective mixture then takes place of the rose-coloured rays with the violet rays, which are more refrangible and more abundant towards the centre of the shadow. The cause is different, but the effect the same.-Comptes Rendus, November 3, 1856, p. 832.

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METEOROLOGICAL OBSERVATIONS FOR NOV. 1856. Chiswick.—November 1. Uniform haze: cloudy. 2. Foggy: cloudy: hazy, 3. Uniformly overcast : very fine. 4. Foggy : very fine. 5. Foggy: light clouds : fine : frosty. 6. Frosty, clear and cold : cloudy. 7. Cloudy: overcast : densely clouded. 8. Densely clouded : rain. 9. Clear : cloudy and fine. 10. Slight rain. 11. Fine, but cold: cloudy and cold. 12. Cloudy: white clouds and clear intervals. 13. Fine, but cold : rain. 14. Clear and cold. 15. Frosty : fine : slight rain. 16. Clear and frosty : fine : sharp frost. 17. Frosty and foggy : dense fog throughout. 18. Slight fog : very fine: rain. 19. Rain : very fine. 20. Densely clouded. 21. Drizzly: hazy: cloudy. 22. Overcast : cloudy and mild. 23. Uniformly overcast : cloudy and fine. 24. Cloudy and fine. 25. Densely overcast : clear, cold, and dry. 26. Hazy : rain and sleet. 27. Very fine. 28. Cloudy : hazy : cloudy. 29. Sharp frost : clear: frosty. 30. Sharp frost: clear : fine. Mean temperature of the month

39°:30 Mean temperature of Nov. 1855

40 .91 Mean temperature of Nov. for the last thirty years

42.93 Average amount of rain in Nov.

2:313 inches. Boston. Nov. 1. Cloudy: rain a.m. 2, 3. Cloudy. 4. Foggy. 5. Cloudy. 6, 7. Fine. 8. Cloudy: rain A.M. and P.M. 9, 10. Fine. 11. Cloudy : rain r.m. 12. Cloudy: rain A.M. and P.M. 13. Cloudy. 14-16. Fine. 17. Cloudy. 18. Fine : rain A.M. 19. Cloudy: rain A.M. 20. Fine. 21. Cloudy. 22. Fine: rain A.M. 23. Cloudy. 24. Fine : rain P.M. 25. Fine. 26. Fine: snow A.M. 27. Cloudy. 28, 29. Fine. 30. Fine: snow P.M.

Sandwick Manse, Orkney.—Nov. 1. Clear, fine A.M.: cloudy, fine P.M. 2. Showers A.m. and P.M. 3. Bright A.M.: cloudy P.M. 4. Cloudy A.m.: clear, fine, aurora P.M. 5. Fog A.M.: cloudy P.M. 6. Hazy, fine A.M.: clear, fine P.m. 7. Hazy, fine A.m.: fine, drops P.M. 8. Showers A.m.: cloudy, drops P.m. 9. Showers A.M.: sleet-showers P.m. 10. Snow-showers A.M.: sleet-showers P.M. 11. Snow.showers A.M.: hail-showers P.M. 12. Cloudy A.M.: clear P.M. 13. Showers A.M.: hail-showers P.M. 14. Snow-showers A.m.: hail-showers P.M. 15. Showers A.M.: snow-showers P.M. 16. Cloudy A.M.: drizzle P.m. 17. Showers A.M. and P.m. 18. Cloudy A.m.: showers P.M. 19. Showers A.m.: cloudy P.M. 20. Cloudy A.M.: drizzle P.M. 21. Drizzle A.M.: rain P.M. 22. Rain A.M.: drizzle P.M. 23. Damp a.m.: drizzle P.M. 24. Drops A.M.: hail-showers, drift P.M. 25. Snow-showers A.M. : cloudy P.M. 26. Cloudy A.M.: rain P.M. 27. Showers A.M.: snow-showers P.M. 28. Hail-showers A.M. and P.m. 29. Hail-showers A.m.; hail-showers, drift P.M. 30. Bright A.m. : hail-showers P.M.

Mean temperature of Nov. for previous twenty-nine years 42°67
Mean temperature of this month

40 .22
Mean temperature of Nov. 1855

43.49 Average quantity of rain in Nov. for previous sixteen years 4:11 inches. The mean temperature of the room in which the barometer is kept was 59°, and the height above the sea-level is 100 feet, so that the observations can be reduced to 32° and sea-level if required.

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Meteorological Observations made by Mr. Thompson at the Garden of the Horticultural Society at CHISWICK, near London ;

by Mr. Veall, at Boston; and by the Rev. C. Clouston, at Sandwick Manse, ORKNEY.

Barometer.

Thermometer.

Wind.

Rain.

Days of

Month.

Chiswick.

Orkney, Sandwick.

Chiswick.

Orkney,
Sandwick.

Boston.
89 a.m.

Boston. 8 a.m.

Chiswick.

1p.m.

Boston.

Sandwick.

Orkney,

Chiswick.

Boston.

1856.
Nov.

Orkney,

Sandwick.

Max.

Min.

9. a.m.

8 p.m.

Max.

M'.

94 a.m. 84 p.m.

Mean.

30*056

29'965

29-65

29'898

29'902

47*8030803866 40-71 39*73)

0-94

1.61

4.61

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