Scientific amusements . considered an embarrassing matter to fixprecisely the value of two lights. Nothing, however, canbe easier in reality, as we will show. In comparingdifferent lights, it is necessary to bear in mind the amountof waste, the colour of the light, the luminous value ofthe source, and the steadiness of the flame. The luminousvalue of a lamp-burner is generally equalled by that of awax candle, and we will take as an example one of thoseat six to the pound. Very precise appliances are used forthis experiment when great exactness is required ; but itis easy to calculate in a simp
Scientific amusements . considered an embarrassing matter to fixprecisely the value of two lights. Nothing, however, canbe easier in reality, as we will show. In comparingdifferent lights, it is necessary to bear in mind the amountof waste, the colour of the light, the luminous value ofthe source, and the steadiness of the flame. The luminousvalue of a lamp-burner is generally equalled by that of awax candle, and we will take as an example one of thoseat six to the pound. Very precise appliances are used forthis experiment when great exactness is required ; but itis easy to calculate in a simple manner the differences inordinary lights. Supposing we desire to test the value oflight given by a lamp and a wax candle, they must both 44 LIGHT. be placed on the table at an equal height, B and A, infront of some opaque body, A, and then a large sheet ofpaper must be fixed as vertically as possible to form ascreen. When B and A are lighted, two shadows, E andF, are produced, to which it is easy to give exactly the. Grotesque efTects of curved surfaces. same intensity, by advancing or withdrawing one of thetwo sources of light. The intensities of the two lightswill then be inversely proportional to the squares of themeasured distances, A B and AC. By a similar carefulcalculation it has been possible to draw up a table of therelative values of various ordinary lights. We have not PHOTOMETRY. 45
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Keywords: ., bookcentury1800, bookdecade189, booksubjectscientificrecreations
