May 2nd, 1887.] 



SCIENTIFIC NEWS. 



length. The wave length of a vibration of light is about 

 4^'vo of an inch. 



Sixteen to eighteen simple vibrations of the air in a 

 second produce the lowest audible sound. The middle C 

 of a piano vibrates 264 times a second. The squeak of a 

 bat has about 40,000 vibrations a second, and is the 

 highest audible note for most ears, many people being 

 unaware that this animal makes any sound but a flutter. 

 More rapid vibrations have no effect upon our senses until 

 a scale of 30 to 31 octaves above this has been ascended, 

 when the vibrations are about sixty-two million millions 

 (62,000,000,000,000) a second. Vibrations of the air at 

 this rapidity would have the minute wave length of j^j^y^j;^ 

 of an inch, which would be altogether too small to affect any 

 nerve, but the wave length of an ether vibrat'on of this 

 frequency is about j^'^ of an inch. It produces the sensation 

 of warmth, and is perceived by the nerves of the skin. 



When a rapidity of about 400,000,000,000,000 is reached, 

 the nerves of the eye are stimulated, and the sensation of 

 red is perceived. The wave length is ,,',- of an inch. 

 600,000,000,000,000 vibrations per second produce green, 

 and 830,000,000,000,000 produce violet. The wave length 

 of violet light is (^^'~ of an inch. The range of audible 

 sounds extends over about eight octaves, but that of colour 

 is almost exactly one octave. Higher vibrations cannot be 

 perceived directly by the eye, but have powerful chemical 

 effect on certain substances, such as those used in photo- 

 graphy. 



By the aid of the colour diagram in the second chapter of 

 Notes on Colour*, we can trace the effects produced by 

 raising a body to a white heat. At first dark heat rays are 

 emitted, then deep red ; and, as was described in the last 

 number of The Scientific News, Capt. Abney has found 

 that there is a great difference among even well-trained eyes 

 as to their perception of the extreme reds. As the body 

 becomes hotter, orange and yellow light is emitted, and by 

 their mixture give a more and more j'ellow light, and since 

 this colour is the brightest of all, the red becomes almost 

 overpowered ; but when the sea-green rays are reached, 

 these with the red make white, and the result is a whitish 

 yellow, and when the blue is emitted it combines with the 

 orange, and finally the violet finds its complement in the 

 green, and a pure white is the result. 



There is a difference of opinion as to whether at a higher 

 temperature the blue and violet rays would be emitted in 

 greater proportion than the red, so that the result would be 

 pale blue ; or whether they are all given off" in the same 

 proportion. It is certain that at the highest temperature of 

 the carbons of an electric arc no pale blue light is percep- 

 tible, the violet tinge which is sometimes seen is due in 

 some cases to impurities, and in others to the colour of the 

 carbon vapour of the arc itself, whose light is but a small 

 fraction of that given off bj'the glowing carbon points which 

 are the real source of light. 



The reverse of this series of changes has been exhibited 

 by Prof. Tyndall in one of his most striking experiments. 

 In a darkened room a powerful beam of light is sent through 

 an empty glass tube, and there being nothing to reflect the 

 rays, the space in the glass appears black. A very small 

 quantity of a volatile vapour is passed in, and the particles 

 gradually increasing in size as it condenses, they become 

 capable of reflecting first the violet rays, and then the blue. 

 As the cloud gathers and thickens, the minute drops become 

 capable of reflecting the green rays, and these combined 

 with the violet, form white, making whitish blue ; and the 

 orange and red rays, being complementary to the blue and 

 sea-green, at last make pure white. 



* See p. 40, ante. 



The different tints of blue in the sky are due to small 

 particles of vapour floating in the air. Overhead, where 

 the layer of air is thinner, the blue is deeper; nearer the 

 horizon the thickness in the direction of sight is greater, and 

 the blue is paler. At high altitudes, from mountain-tops, 

 or from a balloon, the colour of the sky is very deep; the 

 more rare the air, the nearer does the colour approach 

 black. 



In Captain Abney's lecture already alluded to, the reverse 

 experiment is described, and Prof Tyndall's experiment, as 

 described, is itself reversible. The cloud which reflects 

 only violet and blue, is naturally transparent to all the 

 other colours, and these together make pale yellow. Thus 

 while the colours of the reflected light pass from dark blue 

 through light blue to white, the transmitted light changes 

 from white to deep red ; the sum of the two being at any 

 time equal to white. 



The two effects may often be seen together, when thin 

 smoke is seen at one time blue against a dark background, 

 and a moment later orange or brown against a white cloud. 

 Tobacco smoke often appears blue or brown by reflected 

 or transmitted light, as it curls up from the hot ash, but 

 it loses nearly all its colour after partial condensation when 

 puffed from the mouth. 



It is difficult to account for the pale green tint not imfre- 

 quently seen at sunset. It cannot be produced directly by 

 small particles, either by reflection or transmission, and it 

 is probably the effect of the contrast of reds nearer the 

 horizon. It is often easy to study a small portion of a sun- 

 set sky reflected in a pool, with the dark ground as a frame, 

 but even then the horizon tints and the warm hues of the 

 whole scene may affect the judgment. 



Minute particles of vapour in the air between a distant 

 mountain and the observer, act precisely as those overhead, 

 and reflect blue light. 



It is obvious that the colour is more apparent when 

 the background is dark. For this reason we often see in- 

 tense blue shadows sweeping over a landscape bathed in 

 sunshine, and the natural redness of the soil or vegetation, 

 or the complementary effect of neighbouring greens will 

 make them purple. 



Goethe proposed a theory of colour based upon the pro- 

 duction of blues, reds, and yellows by mixtures of black and 

 white, but where it only for the facts that all the tints but 

 the extreme violet and red, which would be very faint, 

 are impure, and that a green is impossible, such a theory 

 is untenable. Its originator probably imagined that greens 

 were easily produced by adding blue to yellow. 



" Those eyes of deep and most expressive blue," and a 

 spot of milk on a tea-tray, owe their tint to no pigment or 

 colouring matter, but to mere turbidity. 



THE CLIMAX MOTOR. 



ON p. 66 we illustrate an arrangement of boiler and 

 engine which possesses some features of interest. It 

 has been designed more especially to supply steam power 

 for minor purposes, and has one important advantage 

 in this respect, inasmuch as it is perfectly safe so far as 

 boiler explosions may be concerned. As the boiler is the 

 chief feature, we will describe that first. Our illustration 

 only gives an outside view, but within the light sheet iron 

 casing shown there are a couple of vertical coils of pipe 

 placed one within the other. At the bottom of the space 

 enclosed by these coils are fire-bars, and the fire is made up 

 in what may be described as the cylindrical cage thus 

 formed. In place of the fuel being supplied through a fire 

 door in the usual way, it is dropped in from the top through 

 the lid shown. The curved pipe at the side is, of course, 



