DR. HUDSON'S REPLY TO DR. APJOHN'S PAPER INSERTED IN THE PHILOSOPHICAL MAGAZINE FOR SEPTEMBER. In the course of September last we received from Dr. H. Hudson a reply, dated Stephen's Green, Dublin, 10th September 1836, to Dr. Apjohn's paper inserted in our Number for that month, accompanied by a private note addressed "To the Editors of the Philosophical Magazine," and dated September 12th, requesting the insertion of the reply. We acknowledged the receipt of Dr. Hudson's communication in the notice "To Correspondents" on the wrapper of our last Number, for October, stating it to be " under consideration". Having now given it full consideration, we regret that from the very personal form which the controversy between Dr. Hudson and Dr. Apjohn has now, perhaps unavoidably, assumed, we feel called upon to terminate the discussion in our pages. We have no intention whatever, in doing this at the present juncture, to express any opinion on the merits of the subject; but were Dr. Hudson's reply to be inserted, Dr. Apjohn would have an equal claim to the publication of his rejoinder, and a controversy, in which nothing would be added to the progress of science, (for the points in dispute do not involve any principles which have not already been fully explained in the original papers), might be continued indefinitely, or we might be compelled to close it at some future stage. Dr. Hudson, however, is entitled to the most explicit record of the promptitude of his reply, and of his contradiction of Dr. Apjohn's statements, on which account we have noted above the reception and date of his communication, and also inserted the present paragraph.-EDIT. FUSELI'S PORTRAIT OF PRIESTLEY. It is not generally known that a portrait exists of Dr. Priestley, painted, when he was about fifty, by the celebrated Fuseli, which derives a value, not only from the interest of the subject, but from the faithfulness of the resemblance and the spirit and excellence of the execution; as well as from the circumstance of its being almost the only portrait which the celebrated artist is known to have painted. That his powers were zealously employed on this picture may be inferred from the circumstance that it was undertaken at his own particular request, and presented to the common friend at whose house they occasionally met, Mr. Johnson, the Bookseller, St. Paul's Churchyard, after whose death it was removed to the Library in Redcross Street. Mr. Turner has been for some time employed upon this portrait, and has produced an excellent engraving from it, for a number of gentlemen who have entered into a subscription for the purpose. He has succeeded in giving to this print, which is a very faithful copy of the picture, a powerful and pleasing effect; and the size is conveniently adapted either for the cabinet or the folio. Subscribers' names are received by Mr. Richard Taylor, at the Office of the Philosophical Magazine, Red Lion Court, Fleet Street, where the copies (Proofs) will be delivered on application. A very few Proofs have been taken before the letters for those who may be desirous of possessing them. SCIENTIFIC MEMOIRS, selected from the Transactions of Foreign Academies of Science, and from Foreign Journals. Part II., just published, contains Researches relative to the Insects, known to the Ancients and Moderns, by which the Vine is infested, and on the Means of preventing their Ravages. By M. le Baron Walckenaer, Hon. Memb. of the Entomological Society of France. The Kingdoms of Nature, their Life and Affinity. By Dr. C. G. Carus, Physician to His Majesty the King of Saxony. Researches on the Elasticity of Bodies which Crystallize regularly. By Felix Savart. Researches concerning the Nature of the Bleaching Compounds of Chlorine. By J. A. Balard. On the Laws of Conducting Powers of Wires of different Lengths and Diameters for Electricity. By E. Lenz. Memoir on the Polarization of Heat. By M. Melloni. cold. showers. at night. 10. Fine. 11. clear and windy at night. haze: fine. 15. Fine. 18-20. Cloudy, and fine. fine. 23. Stormy and wet. heavy showers: fine. METEOROLOGICAL OBSERVATIONS FOR SEPTEMBER 1836. Chiswick.-Sept. 1. Very fine. 2. Overcast : stormy showers: clear and 3. Fine. 4. Rain: fine. 5. Cloudy: very fine. 6. Stormy 7. Fine, but cool. 8. Very fine. 9. Overcast cloudy: rain Cloudy: stormy at night. 12. Stormy showers: 13. Cloudy and cold: boisterous. 14. Cold 16. Showery. 17. Fine: thunder showers. 21. Cold and damp: fine. 22. Foggy: very 24, 25. Fine. 26, 27. Hazy: fine. 28. Clear: 29. Rain. 30. Clear and cool: stormy showers. The summer, late in commencing as regards temperature, may be said to have terminated with the beginning of this month. The temperature falling so early and abruptly was confidently expected to remain only temporarily depressed; but such expectations have been disappointed. 2. Cloudy: rain early A.M.: rain A.M. 3. Fine. 4. Rain rain P.M. 5. Cloudy rain early A.M. rain early A.M. : rain P.M. 7. Cloudy: rain P.M. 8. Fine. 10. Cloudy: rain early A.M.: rain P.M. 11. Cloudy: rain P.M. 13. Rain. 14. Cloudy: rain early A.M.: rain P.M. 15. Cloudy. rain early A.M.: rain P.M. 17. Cloudy. 18. Fine. 20. Cloudy: rain P.M. 21. Fine: rain P.M. 23-25. Fine. 26. Cloudy. 27. Cloudy: rain P.M. 29. Cloudy: rain P.M. 30. Cloudy. Boston.-Sept. 1. Fine. 6. Cloudy: 9. Rain. 12. Stormy. 16. Cloudy. 19. Cloudy: 22. Cloudy: rain P.M. 28. Cloudy. Meteorological Observations made at the Apartments of the Royal Society by the Assistant Secretary; by Mr. THOMPSON at the Gardens of the Days of Chiswick. 56.2 69.6 72 50 61 Sw. SW. calm ... ⚫01 54:4 63.5 67 38 56 sw. var. SW. calm .22 ⚫05 57.1 46.9 64.2 68 48 54 S. S. calm •169 *45 •15 O 4. 29-386 29-472 29:372 28.85 61.8 54.0 68.0 70 51 57 sw. S. calm *463 .14 *38 M. 5. 29.560 29-658 29.538 28-84 56:4 53.5 62.5 47 55 sw. var. w. NW. 113 T. 6. 29-360 29-476 29-303 28-71 56.3 50.9 61.7 64 48 55 SSE. var. SW. W. *091 W. 7. 29.641 29.785 29-611 29.02 53.7 51.7 60.5 63 46 53.5 NW. N. calm •158 Th. 8. 29.780 29-784 29-738 29-21 55.9 52.0 63.6 66 47 55 WNW. Sw. calm ... SW. calm ... 29-27 53-6 46.5 56.8 60 39 42.4 56.7 61 49 986 WNW. w. calm •272 48.5 WSW. W. W. NE. N. NE. 54 NNE. 48.4 59.0 61 44 55 NNE. N. *022 E. 56 NE. NE. calm ... 29.406 28.77 52.5 46.6 54.6 55 34 51 WSW. W. calm 255 24 05 52 54 54 53 50 52 48 43 54 LONDON AND EDINBURGH PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. [THIRD SERIES.] DECEMBER 1836. ΑΙ LXXVI. Facts relating to Optical Science. No. IV. § 1. Experiments on the Interference of Light. LTHOUGH so much has been explained in optical science by the aid of the undulatory hypothesis, yet when any well-marked phænomena occur which present unexpected peculiarities, it may be of importance to describe them, for the sake of comparison with the theory. Such appears to me to be the case with those which I am about to mention, in which, by means of a remarkable compensation of some kind or other, common solar light appears to play the part of homogeneous light, and to achromatize itself, if I may use such an expression, in a very high degree of perfection. Sir William Herschel was, I believe, the first who took notice of the very beautiful coloured bands which are seen by looking through two prisms placed in contact. Thus, let ABC, ADC be two equal right-angled glass prisms in contact. We will suppose the sides A B, BC to be equal, and the thickness of the prisms to be equal to A B, in which case the combination of the two will form a cube. Let the two prisms be gently pressed together by their face A C, which must be previously well cleaned from any adhering dust, and *Communicated by the Author. Third Series. Vol. 9. No. 56. Dec. 1836. 3 B let them be fixed firmly in this position. Then if the observer looks through the cube at a bright white object, or at the sky, he will see a number of coloured parallel bands, the direction of vision being supposed to be perpendicularly through two opposite sides, as A B, CD. If instead of this he looks through A B at a light coming from the direction XX and then reflected internally on the face A C, he will again see numerous coloured bands upon A C, but these will be complementary in their tints to the former B ones. These coloured bands are analogous in their nature to Newton's rings, differing only in being formed between two plane surfaces either parallel or very nearly so, and viewed by the observer at an incidence of 45°. But the beauty of the appearances may be surprisingly increased by transporting the apparatus into a dark chamber, and suffering a single pencil of the brightest solar light to pass through the prism, or to be reflected from the face A C. If then a sheet of white paper be held up, at any distance from the prism, the coloured bands are depicted upon it with the greatest vivacity and distinctness. The transmitted bands have altogether a different character from the reflected ones, so that it is impossible to mistake one for the other, even without reference to the path of the ray. This experiment, easily tried, is one of the most beautiful in optical science; I shall not, however, dwell upon it, because I believe it is sufficiently well known, and that it has been exhibited in some public lectures. Now, in making this experiment with care, I have observed some remarkable circumstances. The coloured bands are not, as has been supposed, isochro matic lines. The deviation is sometimes very marked, so that a band in the course of its progress acquires very different tints from those which it possessed originally. This fact may be considered of some importance with respect to the theory. It takes place when the prisms are in close contact, and the bands few in number. But the following is still more deserving of attention. When the contact of the prisms is diminished by interposing a hair between them, (still pressing them together,) the coloured bands depicted upon the paper, become more numerous, narrow and crowded. Frequently they alter |