three equations to determine ri, r2, and rs the atomic refractions of carbon, hydrogen, and oxygen respectively, and if we find that the refractive indices of other compounds may be determined with the help of the values so found, we shall have justified the above statement. It appears, however, that the way in which an atom is combined affects its refractive power, and to obtain consistent results it is necessary to assume, for instance, that Oxygen in the carbonyl group has a different refractive power making use of the following numbers given by Landolt*. directly. If, for instance, we take three compounds containing carbon, hydrogen, and oxygen in different proportions, we obtain Calculate the refractive index of ethyl alcohol C2HO, from the line i illuminat sump SECTION XLIV. PHOTOMETRY. Apparatus required: Photometer, scale, candles, and gas lamp. The photometer is an instrument for comparing the illuminating powers of different sources of light. One of the most accurate forms of the instrument is that in which the two sources are placed on opposite sides of a screen capable of partly reflecting and partly transmitting the light from each source. When this screen is placed in such a position that the straight lines drawn from it to the two sources make equal angles with the screen, and the illumination of the two sides of the screen appears the same, the amounts of light to be compared are to each other as the squares of the distances of the sources from the screen. Having fixed on the light sent out by some standard source in a given direction as the unit, we can by a comparison of any source with the standard determine the illuminating power of that source. The standard source is a sperm candle flame consuming 120 grains of sperm per hour, and the standard direction any line in a horizontal plane. If the consumption of sperm differs from the standard rate by less than 10 grains per hour, the illuminating power may be taken as proportional to the consumption. Arrange the two sperm candles, the photometer, and the gas flame provided, in a straight line in the order named, the two candles being mounted about 5 cms. apart in a direction perpendicular to the straight line, on a stand capable of moving along a graduated scale parallel to the line, Fig. 94. Place the Fig. 94. gas flame at some convenient fixed distance from the photometer screen, say 1 metre, with a flat surface towards the screen, light the candles and after a few minutes move them backwards and forwards along the scale, till the illumination of the two sides of the photometer screen is equal. If the simple Bunsen screen is used, this is the case when the semiopaque and semi-transparent parts of the screen appear of equal brightness, and if the Lummer-Brodhun instrument1 is used, when the inner and outer parts of the field of view of the telescope through which the screen is viewed appear of equal brightness. If it is not possible to produce equality of brightness without moving the candles inconveniently near to or far from the photometer, increase or decrease the distance of the gas flame. Blow out the candles, weigh and replace them, and at a given instant relight them. Move them till equality of brightness of the screen is produced, and determine the distance of the candles from the screen by reading the scale position of a point vertically under the candles. If the photometer screen can be reversed this should be done and equality of illumination again secured by moving the candles. Their position should again be read on the scale. These observations should be repeated three or four times and the means taken. Rotate the gas flame so that the flat surface is now inclined at 45° to the line drawn to the photometer, and repeat the observations. Rotate the flame again through 45° so that the edge is presented to the photometer, and repeat. Continue 1 This is the instrument shewn in Fig. 94. the rotations and observations till those first taken have been repeated. Blow out the candles, noting the time, and again weigh them. Record as follows: 19 Jan., 1897. Tulip gas-burner. Distance D of gas flame from photometer screen= : 80 cms. 154-2 grams. Represent these results graphically as follows:- Draw a plan of the gas flame as seen from above, and draw from the centre of the plan lines through the edges, and lines inclined at 45° and 90° respectively to the surfaces of the flame. Along each line take a length proportional to the candle power observed in its direction. Join the points so determined by a smooth curve. This is known as a "horizontal photometer curve" for the flame. By taking observations in directions inclined at intermediate angles it can be drawn with greater accuracy. SECTION XLV. INTERFERENCE OF LIGHT. Apparatus required: Bunsen flame, sodium bead, slit, biprism, micrometer microscope, metre scale. When two beams of homogeneous light coming from the same source cross each other after having described paths differing in length, the vibrations due to the two may be in opposite directions and neutralise each other at certain points of the region where the beams cross. At such points the joint action of the two beams will produce partial or total darkness, and if a screen is placed in this region, a series of light and dark "interference" bands will be seen on it. In the case in which the source of light is a narrow vertical slit behind which a sodium flame is placed, and the two beams are produced by the passage of the light from this slit through a double prism the section of which is indicated in Fig. 95, the distance between consecutive dark bands on the screen, is related to the wave-length λ of the light, the distance a of the slit from the screen, and the distance apart c of the two virtual images of the slit Fig. 95. хс formed by the biprism, by the equation: λ=c. α This method of producing interference bands may therefore be used to determine approximately the wave-length of light. The experiment is performed on an "optical bench,” a simple form of which is shewn in Fig. 96. The slit consists of two brass plates with straight bevelled edges, attached by screws to a wooden stand provided with levelling screws. |