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MARCI 22, 1883. J THE PHOTOGRAPHIC NEWS. 183 million-millionth of a second, the radiation can only be per ceived by the sense of heat; when the period of vibration is shorter than one four-hundred-million-millionth of a second, and longer than one eight-hundred -million-mill i ..th of a second, the radiation is perceived as light by the eye. Pouillet, from a series of experiments, deduced a value of the energy radiated by the sun, equal in British units to about 86 foot-pounds per second per square foot at the earth’s surface, or about one horse-power to every 6± square feet of the earth’s surface. We may estimate from this the value of the solar radiation at the surface of the sun. The sun is merely an in candescent molten mass losing heat by radiation, and surrounded by an atmosphere of incandescent vapour, so that the radiant energy really comes out from any square foot or square mile of the sun’s surface, as from a pit of luminous fluid which we can cannot distinguish as either gaseous or liquid. Take, however, instead of the sun, an ideal radiating surface of a solid globe of 440,000 miles radius. The distance of the earth being taken as 93,000,000 miles, the radius of the sun is equal to, say in round numbers, one two-hundredth of the earth’s distance, hence the area at the earth’s distance corresponding to one square foot of the sun’s surface, is equal to 40,000 square feet. The radiation on this surface is (40,000 X 86, or) 3,440,000 foot-pounds, which is therefore the amount of radiation from each square foot of the sun’s surface. This amounts to about 7,000 horse-power, which, according to our brain-wasting British measure, we must divide by 144, if we wish to know the radiation per square inch of the sun s surface, which we thus find to be 50 horse-power. The normal current through a Swan lamp giving a 20-candle light is equal to 1-4 amperes with a potential of 40 to 45 volts. Hence the activity of the electric working in the filament is 6T6 ampere-volts or Watts (according to Dr. Siemens’ happy desig nation of the name of Watt, to represent the unit of activity constituted by the ampere-volt). To reduce this to horse-power, we must divide by 746, and we thus find about 1-12th of a horse-power for the electric activity in a Swan lamp. The filament is 32 inches long, and *01 of an inch in diameter of circular section ; the area of the surface is thus 1-9th of a square inch, and therefore the activity is at the rate of 3-4ths of a horse-power per square inch. Hence the activity of the sun’s radiation is about sixty-seven times greater than that of a Swan lamp per equal area, when incandesced to 240 candles per horse power. In this country the standard light to which photometric measurements are referred is that obtained from what is known as a standard candle. Latterly, however, objections have been raised against its accuracy. It has been said that differences of as much as 14 per cent, have been found in the intensity of the light given by different standard candles, and that serious differences have been observed in the intensity of the light from different parts of the same candle in the course of its burning. The Carcel lamp, the standard in use in France, has been re regarded as the only reliable standard. It is, no doubt, very reliable and accurate in its indications, but it should be remem bered that its accuracy is greatly owing to the careful method and the laborious precautions taken to secure accuracy. If some thing akin to the precautions applied to the Carcel lamp by Regnault and Dumas were applied to the production and use of the standard candle, there is little doubt but that sufficient accuracy for most practical purposes could also be obtained with it; probably as good results as are already obtained by the use ol the Carcel lamp. At the Conference on Electrical Units which met in Paris lately, a suggestion was made to use as a standard for photo metric measurements the incandescence of melting platinum, and very interesting results and methods in connection with the proposal were presented to the meeting. According to experi ments by Mr. Violle, which M. Dumas reported to the Confer ence, a square centimetre of liquid platinum at the melting temperature gives of yellow light seven, and of violet twelve times, the quantities of the same colours given by a Carcel lamp. The apparent area of the Swan filament, being one-ninth of a square inch, is *23 of a square centimetre, and when incandesced to 20 candles, must be about as bright as the melted platinum of Mr. Violle’s experiment, as the 7 carcels of yellow and 12 of violet must correspond to something like 10 carcels or 85 candles, in the ordinary estimation of illumination by our eyes. The tint of Mr. Violle’s glowing platinum cannot be very different from that of the ordinary Swan lamp incandesced to its « 20 candles.” Thus both, as to tint, and brightness, it appears that melted platinum at its freezing temperature is nearly the same as a carbon filament in vacuum incandesced to 240 candles per horse-power. For approximative photometric measurements the most con venient method is certainly that of Rumford, by a comparison of the shadows cast by the sources of light on a white surface. The apparatus necessary are only a piece of white paper, a small cylindrical body such as a pencil, and a means of measuring distances. Ordinary healthy eyes are usually quite consistent in estimating the strength of shadows, even when the shadows examined are of different colours, and with a reasonable amount of care photometic measurements by this method may be obtained within 2 or 3 per cent, of accuracy. The difference in the colours of the shadows is of course due to each shadow being illuminated by the other light. Arago has compared the luminous intensity of the sun with that of a candle, and estimates it as equal to about 15,000 times that of a candle flame. Seidel, as Sir W. Thomson had been informed by Helmholtz, estimated the luminous intensity of the moon as about equal to that of grayish basalt cr sandstone. An experiment on sunlight made in Glasgow on the 8 th of this month (since this paper was read), compared with an observation on moonlight, which he made at York during the meeting of the British Association there in 1881, had led him to conclude that the surface of the moon radiates something not enormously different from one- quarter of the light incident on it. It would be exactly this if the transparency of the Glasgow noon atmosphere of Decembers 1882, had been exactly equal to that of the York midnight atmosphere of September, 1881, referred to below, for the respective altitudes of the sun and moon on the two occasions. The observation on moonlight referred to above showed the moonlight at the time and place of the observation (at York early in September, 1881, about midnight, near the time of full moon) to be equal to that of a candle at a distance of 230 centimetres. The moon’s distance (38 X I0 10 cm.) is 1-65 X 108 times the distance of the candle. Hence, ignoring for a moment the loss of moonlight in transmission through the earth’s atmosphere, we find (1'65 X 10 8 ) 2 , or 27 thousand million million as the number of candles that must be spread over the moon’s earth ward hemisphere painted black, to send us as much light as we receive from her. Probably about one and a-half times as many candles, or say forty thousand million million would be required, because the absorption by the earth’s atmosphere may have stopped about one-third of the light from reaching the place where the observation was made. The moon’s diameter is 3 5 X 10 8 centimetres, and therefore half the area of her surface is 19 X 10 16 square centimetres, which is nearly five times forty thousand million million. Thus it appears that if the hemi sphere of the moon facing the earth were painted black and covered.with candles standing packed in square touching one another (being, say, one candle to every five square centimetres of surface), all burning normally, the light received at the earth would be about the same in quantity as estimated by our eyes, as it really is. It would have very much the same tint and general appearance as an ordinary theatrical moon, except that it would be brightest at the rim and continuously less bright from the rim to the centre of the circle where the brightness would be least. The luminous intensity of a cloudy sky he found about 10 a.m. one day in York during the meeting of the British Association to be such, that light from it through an aperture of one square inch area was equal to about one candle. The colour of its shadow compared with that from a candle was as deep buff yellow to azure blue, the former shadow being illuminated by the candle alone, the latter by the light coming through the inch hole in the window shutter. The experiment on sunlight of last Friday (December 8) showed, at 1 o’clock on that day, the sunlight reaching his house in the University to be of such brilliancy that the amount of it coming through a pinhole in a piece of paper of *09 of a centimetre diameter produced an illumination equal to that of 126 candles. This is 6’3 times the 20-candle Swan light, of which the apparent area of incandescent surface is "23 of a square centimetre, or 3’8 times the area of the pin-hole. Hence the sun’s surface as seen through the atmosphere at the time and place of observation was 24 times as bright as the Swan carbon when incandensced to 240 candles per horse-power. By cutting a piece of paper of such shape and size as just to eclipse the flame of the candle and measuring the area of the piece of paper, he found about 2’7 sq. centims. as the corre sponding area of the flame. This is 420 times the area of the