404 



NATURE [March 21, 1878 



Tension of the Globe of the Eye in Man and Animals, and on 

 the Physiological Influence on this Tension of such Substances 

 as A tropia, Daturin, Eserine, and Pilocarpine £2t^ 



Dr. C. R. A. Wright, St. Mary's Hospital, Paddingtoa— For 

 Continuation of Researches on certain Points in Chemical 

 Dvnamics ; on the Determination of Chemical Affinity in 

 Terms of Electrical Magnitudes ; and on some of the lesser- 

 known Alkaloids ... ;^200 



Prof. Schorlemmer, Owens College, Manchester. — For Continua- 

 tion of Researches into (i) the Normal Paraffins, (2) Suberone, 

 (3^ Aurin ^150 



W. N. Hartley, King's College, Strand. — For Investigation of 

 the Fluid Contents of Mineral Cavities ; of the Properties of 

 the Phosohate of Cerium ; of Methods of Estimating the 

 Carbonic Acid in small Samples of Air ; and of Photographic 

 Spectra ^150 



Dr. Armstrong, Lewisham Village, S.E. — For Continuation of 

 Researches into the Phenol Series ^250 



THE SOURCES OF LIGHTS 



VXTHEN the sun rises in the morning, the darkness of 

 '' * the night seems to fade away, and, wherever we 

 look, without or within, all the air and space about us 

 appears to be full of light. When evening comes again, 

 the daylight disappears, and the moon and the stars give 

 us another hght. In the house we start the lamps, and 

 they give us another Hght. Out-of-doors, in the dusky 

 meadows, we see the fire-flies darting about, and giving 

 out pale sparkles of yellow light as they fly. We look to 

 the north in the night and see the aurora, or we watch 

 the lightnings flash from cloud to cloud, and again we see 

 more light. 



This light from sun and moon, the stars, the fire, the 

 clouds, the sky, is well worth studying. It will give us a 

 number of the most beautiful and interesting experiments, 

 and by the aid of a lamp, or the light of the sun, we can 

 learn much that is both strange and curious, and perhaps 

 exhibit to our friends a number of charming pictures, 

 groups of colours, magical reflections, spectres,and shadows. 

 All light comes from bodies on the earth or in the air, or 

 from bodies outside of the atmosphere ; and these bodies 

 ■we call the sources of light. Light from sources outside 

 of the atmosphere we call celestial light, and the sources 

 of this light are stars, comets, and nebulse. The nebulae 

 appear like flakes and clouds of light in the sky, and the 

 comets appear only at rare intervals, as wandering stars 

 that shine for a little while in the sky and then disappear. 

 The stars are scattered widely apart through the vast 

 spaces of the universe, and they give out their light both 

 driy and night. The Isrightest of these stars is the sun. 

 when it shines upon u-, the other stars appear to be lost 

 in the brighter light of this greater star, and we cannot see 

 them. At night, when the sun is hid, these other stars 

 appear. We look up into the sky and see thousands of 

 them, fixed points of light, each a sun, but so far away 

 that they seem mere spots and points of light. Besides 

 these stars are others, called the planets, that move round 

 the sun. These give no light of their own, and we can 

 only see them by the reflected light of the great star in the 

 centre of our solar system. Among these stars are the 

 Moon, Venus, Mars, Jupiter, and many others. We might 

 call celestial light starlight ; but the light from the great 

 star, the sun, is so much brighter than the light of the 

 others, that we call the light it gives us sunlight, and the 

 light from the other suns we call starlight. For conve- 

 nience, we also call the reflected light from the planets 

 starlight, and the light from our nearest planet we call 

 moonlight. 



Terrestrial'light includes all the light given out by 

 things on the earth, or in the air that surrounds the earth. 



* From a forthcoming volume of the " Nature Series "— " Light : a Series 

 of Simple. Entertaining, and Inexpensive Experiments in the Phenomena 

 of Light, for the Use of Students of Every Age," by Alfred M. Mayer and 

 Charles Barnard. 



The most common light we call firelight, or the light that 

 that comes from combustion. When we light a lamp or 

 candle, we start a curious chemical action that gives out 

 light and heat. The result of this action is fire, and the 

 light that comes from the flame is firelight. When a 

 thunder-storm rises, we see the lightning leap from the 

 clouds, and give out flashes of intensely bright light. 

 Sometimes, at night, the northern sky is full of red or 

 yellow light, darting up in dancing streamers, or resting 

 in pale clouds in the dark sky. You have seen the tiny 

 sparkles of light that spring from the cat's back when you 

 stroke her fur in the dark, or have seen the sparks that 

 leap from an electrical-machine. All these — the aurora, 

 the lightning, and the electric sparks — are the same, and 

 we call such light electric light. 



Sometimes, in the night, we see shooting-stars flash 

 across the sky. These are not stars, but masses of 

 matter that, flying through space about the earth, 

 strike our atmosphere and suddenly blaze with light. 

 The friction with the air as they dart through it is so great 

 that these masses glow with white heat, and give out 

 brilliant light. Two smooth white flint pebbles, or two 

 lumps of white sugar, if rubbed c|uickly together, will 

 give out light, and this light we call the light from 

 mechanical action. 



Sailors upon the ocean sometimes see, at night, pale- 

 yellow gleams of light in the water. A fire-fly or glow- 

 worm imprisoned under a glass will show, in the dark, 

 bright spots of light on his body. A piece of salted fish 

 or chip of decayed wood will sometimes give a pale, cold 

 light in the night-; and certain chemicals, like Bologna 

 phosphorus and compounds of sulphur, lime, strontium, 

 and barium, if placed in the sunlight in glass vessels and 

 then taken into the dark, will give out dull-coloured lights. 

 All these — the drops of fire in the sea, the glow-worm, the 

 bit of decayed wood, and these chemicals — are sources 

 of the light called phosphorescence. 



These are the sources of light — the stars, the fire, 

 electricity, friction, and phosphorescent substances. We 

 can study the light from all of them, but the light from 

 the sun or a lamp will be the most convenient. The light of 

 the sun is the brightest and the cheapest light we can find, 

 and is the best for our experiments. A good lamp is the 

 next best thing, and in experimenting we will use either 

 the sun or a lamp, as happens to be most easy and con- 

 venient. 



The Heliostat. 



In looking out of doors in the daytime we find that the 

 sunlight fills all the air, and extends as far as we can see. 

 It shines in at the window and fills the room. Even on 

 a cloudy day, and in rooms where the sunshine cannot 

 enter, the light fills everyth ng, and is all about us on 

 every side. Now, in studying light we do not wish a 

 great quantity. We want only a slender beam, and we 

 must bring it into a dark room, where we can see it and 

 walk about it and examine it on every side, bend it, split 

 it up into several beams, make it pass through glass or 

 water, and do anything else that will illustrate the laws 

 that govern it. 



Choose a bright, sunny day, and go into a room having 

 windows through which the sun shines. Close the 

 shutters, curtains and blinds, at all the windows save one. 

 At this window draw the curtain down till it nearly closes 

 the window, and then cover this open space with a strip 

 of thick wrapping-paper. Cut a hole in this paper about 

 the size of a five-cent piece, and at once you will have a 

 slender beam of sunlight entering the hole in the paper 

 and falling on the floor. Close the upper part of the 

 window with a thick shawl or blanket, and, when the 

 room is perfectly dark, our slender beam of light will 

 stand out clear, sharp, and bright. 



As soon as we begin to study this beam of light, we 



find two little matters that may give us trouble. The sun 



j does not stand still in the sky, and our beam of light 



