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THE temperature of the sun cannot be measured by the methods with which we are familiar in our every day life; some other method must be devised. The heat and light of the sun reach us by a process which we call radiation. Of the three processes by which heat may be transmitted from one place to the other conduction and convection are well known, and instances abound of the part they play in heat transference; but we are not so familiar with the method of radiation. How does the energy from the sun reach us across the ninety-two million miles of so-called empty space between earth and sun? The view that the sun could give us heat and light without anything happening in the space between us was abhorrent to experimental philosophers and a theory was promulgated that this so-called empty space is filled with a medium called 'ether.' The only laws which this ether obeys are the laws which are necessary for the passage of transverse wave-motion through it. The solar radiant energy is brought to us by this motion. The same laws apply to radiation as to light. 'Substance of a lecture given to the Society on Dec. 10th, 1918

In fact, the physical processes of radiation and light are identical, light being just that portion of the radiation which excites the sensation of vision. There is a complete parallelism of experiments on 'light' and 'radiation' as far as propagation, reflection, refraction, dispersion, diffraction and polarization are concerned. We usually do the experiments with the 'light' because we can 'see' the results. In the case of reflection by a concave reflector, the 'light' focus is also the 'heat' focus. In the case of an ordinary burning glass the same identity occurs, except in so far as chromatic aberration pushes the 'heat' focus a little farther away from the lens than the light focus. By using a prism and one or more lenses we can throw a spectrum of the sun's radiation upon the screen. We only see a band of colors on the screen, ranging from violet to red, but it is important to remember that what we see is but a small portion of the whole of the spectrum. If we use lenses and prisms which do not absorb the radiation, and also suitable 'detectors, it is possible to observe the presence of radiation beyond the violet, and also beyond the red. All the waves which fall upon the screen have the same mechanical basis; the only difference is that of wave-length. We have an analogy in sound, where all the sound waves given out by a musical band travel with the same velocity, but our ears distinguish one from the other by the difference in pitch. There is a wide variation of wave length in the radiation from a body like the sun, and the spectrum looks almost continuous. Some heated vapors, however, give out radiation of definite wave-lengths and in these cases the spectrum consists of a number of bright lines, each bright line being the image of the illuminated slit, produced by radiation of some definite wavelength. The red rays have a wave-length of about seven tenthousands of a millimetre, or 7 micron, while for the violet the wave-length is about 4 micron.

The other colors range in between these. Waves of length less than that of violet light are called ultra-violet waves. They do not excite vision, but they are chemically active and affect photographic plates. Waves longer than seven-tenths of a micron are known as infra-red. They extend over a very wide range; they

have little chemical activity, but some have considerable heating effect. Of course, all the waves carry energy; it is only their manifestations that differ. Whenever the energy of the waves (whether ultra-violet, lumionus, or infra-red) is absorbed, heat is developed and by the theorem of the conservation of energy the heat developed is a measure of the energy in the waves. This gives us the basis of the usual method of measuring radiant energy.

It was found in the early days of science that black surfaces, e.g., one covered with lamp-black, absorbed practically all the wave-energy incident on them, and most of the measuring instruments have their sensitive receiver coated with a dead-black surface. One of the first experimenters was Leslie, who used his differential thermoscope for this purpose. This consisted of two glass bulbs connected by a U-tube containing a liquid index. One bulb was blackened and when the instrument was exposed to a stream of radiant energy, the blackened bulb became the warmer of the two and the index moved away from it. Good work was done by Leslie with this instrument. In time it was superseded by Nobili's thermopile, in which was used the principle discovered by Seebeck, that, if a circuit is made of two metals and one junction is heated to a higher temperature than the other, a current of electricity flows around the circuit. This current can be measured by inserting a delicate galvanometer in the circuit. It may be increased by using many junctions, exposing alternate ones to the radiation and keeping the others cold. The current is proportional To the difference of temperature between the hot and cold junctions as long as this difference is small. Also the difference of temperature is proportional to the heat absorbed by the junctions from the stream of radiation, and hence the current is proportional to the intensity of the radiation. This method of measuring energy streams has been widely used and can be made very sensitive. D'Arsonval and Boys used it in the radiomicrometer to measure total energy. In conjunction with a large mirror, Boys used this apparatus to measure the radiation from a candle three miles away. Rubens, by using a number of tiny junctions in a

row, has measured the energy sent into the different 'lines' of the spectrum.

Another measuring device is based upon the alteration of the electrical resistance of metals, usually platinum, by a change of temperature. This alteration is about one-third of one per cent. per degree Centigrade. Langley made up a grid of very fine platinum strips and used a Wheatstone bridge arrangement to measure the resistance. The apparatus was sensitive enough to measure a rise of temperature of 1/10000° C. Lummer and Kurlbaum have increased the sensitiveness by using a differential arrangement of grids, and have also modified the construction to nieasure the radiation sent into a spectrum line.

The first man to show that the sun's radiation extended beyond the range of the visible spectrum was W. Herschel. He threw a solar spectrum on the screen and by moving a thermometer from one end to the other, he showed by the heating effect on the thermometer the existence of the infra-red rays. Later work has shown that this infra-red radiation vastly extends the solar spectrum. In the case of a line spectrum such as is observed with a mercury lamp, one must remember that each spectrum line on the screen is an image of the illuminated slit produced by radiation of some definite wave-length, the wave-length corresponding to the position of the line on the screen. A continuous spectrum means that the light going through the slit has an infinite gradation of wave length. A dark line in a spectrum, e.g., in the solar spectrum, indicates the absence of radiation of some particular wavelength, or rather the very low intensity of such radiation, so low that in comparison with its surroundings the line appears dark. The low intensity is due to absorption by the outer and therefore colder layers of the solar atmosphere. By taking a linear thermopile or bolometer through the spectrum from end to end, the distribution of energy in the spectrum can be measured. This has been done by Langley, Lummer and Kurlbaum, and others, and by adding up all the energy, the total energy has been obtained. which should, of course, check up with the measurement of total energy made when the dispersing prism is not interposed. There

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