. Annals of Philosophy. '824.] Mr. Powell on Solar Light and Heat. 93 (44.) Observations of the heat exterior to the cone of light formed by a lens. Aperture, 3*25 inches ; focal length, 7*5. Bulb of photometer coated with Indian ink, in glass case. Distance without the Distance from lens. ... ........ Indication of photometer. rays. Exper. 1. Exper. 9. $ inch. 7-5 inch (at focus) 6 4 2 Close under lens. 17° 12° 12° 10° 9° 8° 7° go 70 16° 12» 11° 9° 8° 70 5 inch. 7*5 inch (lens covered) 5° 4° 1 -25 inch (under the ) furthest part of the > shadow). j 7 "5 (lens open) 40 The two last obs
. Annals of Philosophy. '824.] Mr. Powell on Solar Light and Heat. 93 (44.) Observations of the heat exterior to the cone of light formed by a lens. Aperture, 3*25 inches ; focal length, 7*5. Bulb of photometer coated with Indian ink, in glass case. Distance without the Distance from lens. ... ........ Indication of photometer. rays. Exper. 1. Exper. 9. $ inch. 7-5 inch (at focus) 6 4 2 Close under lens. 17° 12° 12° 10° 9° 8° 7° go 70 16° 12» 11° 9° 8° 70 5 inch. 7*5 inch (lens covered) 5° 4° 1 -25 inch (under the ) furthest part of the > shadow). j 7 "5 (lens open) 40 The two last observations show how much of the effect is to be attributed to reflected light. (45.) The experiment of Sir W. Herschel, from which a maxi- mum heating effect further from the lens than the focus of greatest light is inferred, will be found in the Phil. Trans. 1800, No. 15, It there appears that sealing-wax was scorched in the same time in the focus, and at half an inch further from the lens ; whilst at half an inch nearer, no effect was produced in double the time. It can, perhaps, scarcely be inferred, that this effect is due to the same cause as that which operates outside of the luminous cone; since it is obvious, that beyond the focus the light again diverges, and we cannot with certainty distinguish the effects due to light under the peculiar modifications to which it may there be subjected, from those which may arise from some peculiar development of heat in the same position. The mere inspection of the adjoining diagram will illustrate the di- rections which the differently coloured rays, separated by the dispersive power of the lens, are made to assume ; and with their different combinations it is highly probable that very different heating effects are produced. This is a topic of great interest, and one which, if more thoroughly examined, seems likely to lead to a more complete acquaintance than We at present possess with the nature of the heating e
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