SCIENTIFIC NE\VS. 



[March 1st, 18&7. 



will be readily understood by referring to Fig. 2. In the 

 Otto engine (Fig. 5) A is the water-jacketed cyhnder ; B 

 the piston when at the end of its in-stroke ; and C a con- 

 tinuation of the cylinder forming a chamber in which the 

 charge is compressed before ignition. On the out-stroke of 

 the piston, gas and air are drawn into the cylinder through 

 the port I, and on the return of the piston the charge is 

 compressed, and then ignited through a little port or touch- 

 hole in the slide M. As soon as the ignition takes place 

 there is a rapid evolution of heat from the burning gases, 

 and this induces a great increase of pressure, which acts on 

 the piston and causes it to do useful work. As regards the 

 efficiency of gas engines, Mr. Clerk tells us that in i860 it 

 was only 4 per cent., and that at the present day in the 

 best compression engines it is 18 per cent. By this he 

 means, that the early engine was only able to convert 4 out 

 of every 100 heat units given to it into mechanical work, as 

 developed in the cylinder ; and that the modern compression 

 engine can give 18 out of every 100 units as indicated work. 

 This result is already better than can be obtained with the 

 best steam engine, but Mr. Clerk very rightly urges fur- 

 ther improvements, and points out where they can best be 

 effected. 



Other questions of interest are discussed, but we have 

 sufficiently indicated the general purport of this useful little 

 book, which we have pleasure in recommending to all who 

 care to study the theory and practice of the gas engine — a 

 motor so many believe capable of eventually supplanting the 

 steam engine. The author's style is clear and concise, and 

 the subject-matter is well-arranged, so that it is not only 

 easy to follow the arguments on which the theoretical effi- 

 ciency of the gas engine depends, but also to compare the 

 various t3'pes of engines actually used. 



NOTES ON COLOUR. 



MR. GLADSTONE remarks in a recently pubhshed 

 article, that "the perception of colour by the eye 

 tells us only of our impression of the thing, not at all of the 

 thing itself" Philosophers also instruct us that light and 

 colour have no existence apart from the eye which sees. 

 When we speak of the redness of a rose, we really mean 

 that its petals have the property of reflecting certain rays 

 of light, and that the Hght produces in us the sensation we 

 call redness. 



It used to be thought that " corpuscles " of light were 

 thrown off, in a manner similar to that in which we believe 

 the rose throws off minute particles which cause the sensa- 

 tion of smell ; a red rose would, according to this, throw 

 off red particles, and a yellow rose, yellow particles. It 

 must have been obvious, one would think, that in general 

 the light is reflected ; that is to say, the source of light is 

 not in the rose, but in the sun or lamp that illumines it. It 

 was not, however, until Newton showed that ordinary 

 white light can be split up into many colours, that it was 

 easy to comprehend that the red colour of the rose is due 

 to the property which it has of selecting the red, and re- 

 flecting it, while the other colours are lost. 



There are two very common errors on the subject of 

 " primary colours." One is, to suppose that Sir Isaac 

 Newton discovered seven primary colours when he spht up 

 a beam of white hght by means of a glass prism. It is an 

 accident, arising, perhaps, from the poverty of our vocabu- 

 lary as compared with the profusion of the tints. The so- 

 called seven colours are, as is well known, violet, indigo, 

 blue, green, yellow, orange, and red. The ordinary colour 

 of indigo is not pure enough to rank among these. A more 

 correct and complete list would be : violet, violet-blue, 

 blue, peacock-blue, sea-green, green, yellow-green, yellow. 



orange-yellow, orange, orange-red, red, deep-red. Most 

 apparently pure tints are a mixture of a group of three or 

 four of these colours. The green of a leaf contains a small 

 amount of violet and blue, and a considerable quantity of 

 deep-red. The orange of a certain kind of nasturtium is 

 nearly pure, and so is the blue of a young larkspur These 

 tints, however, are not more beautiful on accunrt of their 

 purity, for exactly the same apparent tint could be pro- 

 duced by a mixture of several colours. Pure colours are 

 very rare in nature. 



There are many seeming paradoxes in the mixtures of 

 colours, for by this expression a mixture of pigments or 

 paints is generally understood, and not a mixture ofrays of 

 light. 



When white light falls on a piece of coloured glass, some 

 of it is reflected at the surface, and the rest attempts to 

 penetrate the glass. Some of the rays succeed in getting 

 through, and others are stopped ; something, however, 

 must become of them ; they are neither reflected nor trans- 

 mitted, they are dissipated in the form of heat. Those 

 rays which get through, have little difficulty in piercing 

 another thickness, they have, as it were, a passport for that 

 substance. Before the light passes through it meets with 

 the second surface, and here some of it is reflected and re- 

 traverses the substance, a portion being again reflected at 

 the first surface- The greater part of that which has not 

 been absorbed passes through. 



In a mass of crushed glass, the light will behave in this 

 manner in passing the first particle, and on striking the 

 next some of it will be reflected, and pass back through one 

 of the upper pieces ; some will strike deeper, and be re- 

 flected through several layers. The general light reflected 

 from the whole mass will be partly white light and partly 

 coloured. This state of things exists in all coloured bodies : 

 they are all more or less transparent. It will be noticed 

 that the less transparent bodies, such as turquoise, or ver- 

 milion paint, have a whitish colour, as compared with that 

 of a sapphire or carmine paint, which are very transparent. 

 It is now clear that in mixing paints, we do not add the 

 colours, but subtract them ; for the light which has passed 

 through the first particle has, in all probability, to return 

 through another of a different kind, and only that light suc- 

 ceeds in traversing both which can pierce both substances. 

 Gamboge absorbs or arrests the violet, blue, and deep- 

 red rays, and allows a considerable quantity of green and 

 orange light to pass. Prussian blue, on the other hand, 

 stops the yellow, orange, and red light, and the only colour 

 which can run the gauntlet of both pigments is the green. 

 If a blue, which contains little or no purple, such as cobalt, 

 and a red, such as vermilion, be mixed, the result will not 

 be purple, but an almost neutral grey, whose lightness is 

 due to the considerable amount of white light which is 

 reflected by each of these paints. 



Colours are said to be simple when they are to be found 

 among those into which white light is split by a prism. 

 There are a large number of tints which are composed of a 

 pure colour and white, such as pink, lemon-yellow, tur- 

 quoise-blue, and a corresponding set of mixtures with black, 

 such as the browns, sage green, and indigo. One colour 

 remains, which is not a simple colour, nor a mixture with 

 white or black : this is purple, which is a compound of blue 

 or violet and red. It has been calculated by Aubert that 

 there are at least one thousand distinguishable simple 

 colours, and about a hundred variations of the strength of a 

 colour, and twenty different mixtures with white, making, 

 together with the purples, browns, slate colour, and 

 other impure mixtures, between two million and three mil- 

 lion different tints. What a field for the artist and the 

 milliner ! Roget only gives about one hundred and seventy 



