Light 



SCIENTIFIC SIDE-LIGHTS 



396 



are within the maps of the universe. 

 TYLOR Anthropology, ch. 13, p. 326. (A., 

 1899.) 

 1932. LIGHT, DEPOLARIZATION OF 



A New Solid Interposed Scatters Dark- 

 ness. When the tourmalins [tourmalin 

 prisms placed in the path of a ray of light] 

 are crossed, the space where they cross each 

 other is black. But the least obliquity on 

 the part of the crystals permits light to get 

 through both. Now suppose, when the two 

 plates are crossed, that we interpose a third 

 plate of tourmalin between them, with its 

 axis oblique to both. A portion of the light 

 transmitted by the first plate will get 

 through this intermediate one. But, after 

 it has got through, its plane of vibration is 

 changed: it is no longer perpendicular to 

 the axis of the crystal in front. Hence it 

 will get through that crystal. Thus, by pure 

 reasoning, we infer that the interposition of 

 a third plate of tourmalin will in part 

 abolish the darkness produced by the per- 

 pendicular crossing of the other two plates. 

 I have not a third plate of tourmalin; but 

 the talc or mica which you employ in your 

 stoves is a more convenient substance, which 

 acts in the same way. Between the crossed 

 tourmalins I introduce a film of this crys- 

 tal with its axis oblique to theirs. You see 

 the edge of the film slowly descending, and, 

 as it descends, light takes the place of dark- 

 ness. The darkness, in fact, seems scraped 

 away, as if it were something material. This 

 effect has been called, naturally but improp- 

 erly, depolarization. TYNDALL Lectures on 

 Light, lect. 3, p. 122. (A., 1898.) 



1933. LIGHT, DOUBLE REFLEC- 

 TION OF, FROM FILMS Bubbles as Teach- 

 ers of Science Extinction of Waves by In- 

 terference Gives Prismatic Colors. Whence, 

 then, are derived the colors of the soap- 

 bubble? Imagine a beam of white light im- 

 pinging on the bubble. When it reaches the 

 first surface of the film, a known fraction of 

 the light is reflected back. But a large por- 

 tion of the beam enters the film, reaches its 

 second surface, and is again in part reflected. 

 The waves from the second surface thus turn 

 back and hotly pursue the waves from the 

 first surface. And, if the thickness of the 

 film be such as to cause the necessary re- 

 tardation, the two systems of waves inter- 

 fere with each other, producing augmented 

 or diminished light, as the case may be. 



But, inasmuch as the waves of light are 

 of different lengths, it is plain that, to 

 produce self-extinction in the case of the 

 longer waves, a greater thickness of film is 

 necessary than in the case of the shorter 

 ones. Different colors, therefore, must ap- 

 pear at different thicknesses of the film. 

 TYNDALL Lectures on Light, lect. 2, p. 66. 

 (A., 1898.) 



1934. LIGHT, ELECTRIC Advantages 

 of Oxygen Not Consumed Air Not Viti- 

 ated Science Increases Healthfulness and 

 Safety. Every other artificial source of 



light, whether gas, or candles, or oil, takes 

 out of the air the oxygen which is necessary 

 for the support of life, and gives back, in 

 return, carbonic acid, which tends to pro- 

 duce suffocation; whereas the incandescent 

 lamp takes nothing from the air, and it 

 gives nothing to it but pure and simple 

 light. Again, the incandescent lamp pro- 

 duces far less heat . . . for a given 

 amount of illumination than other sources 

 of light. Once more, oil and candles and gas 

 often produce a disagreeable smell, and al- 

 ways produce more or less smoke, which dis- 

 colors the walls and ceilings of your rooms, 

 injures your paintings and the bindings of 

 your books, and disfigures every kind of 

 decorative work. The incandescent lamp 

 produces no smoke, and what to many is, 

 perhaps, even more important, it produces 

 no smell. 



A very remarkable testimony to the 

 healthfulness of the incandescent lamp, as 

 compared with gas, was given by Mr. Preece, 

 at the meeting of the British Association 

 recently held in Bath. About two years ago, 

 the electric light was introduced into the 

 Central Post Office Savings Bank in London, 

 and since that time the leaves of absence, 

 on account of illness, of members of the staff 

 have been reduced by an amount equal to an 

 average of two days a year for each person. 

 This, he said, was equivalent to a gain to 

 the service of the time of eight clerks, and 

 represented a saving of about 640 a year 

 in salaries. 



As regards the danger of fire, it is not 

 easy to exaggerate the extraordinary safety 

 of the incandescent lamp. I would only call 

 your attention to one fact. In dealing with 

 gas and candles we are dealing with a naked 

 flame, whose function it is to set fire to 

 whatever touches it; in the case of the in- 

 candescent lamp we are dealing with a light 

 shut up in a prison-house of glass, and if we 

 chance to break the glass we at the same 

 moment put out the light. MOLLOY The 

 Electric Light, lect. 2, p. 37. (Hum., 1889.) 



1935. 



Scarcely Affects 



Germs. It has been found that the electric 

 light has but little action upon bacteria, 

 tho that which it has is similar to sunlight 

 [i. e. } destructive]. Recent experiments 

 with the Rontgen rays have given negative 

 results. NEWMAN Bacteria, ch. 1, p. 25. 

 (G. P. P., 1899.) 



1936. LIGHT ESSENTIAL TO 

 GROWTH OF PLANTS Sun the Source of 

 Plant-life. Light, which is now known to 

 modify many inorganic compounds light, 

 which works those mechanical changes 

 utilized in photography, causes the combina- 

 tions of certain gases, alters the molecular 

 arrangements of many crystals, and leaves 

 traces of its action even on substances that 

 are extremely stable may be expected to 

 produce marked effects on substances so 

 complex and unstable as those which make 

 up organic bodies. . . . The molecular 



