Dec. 30, 1S81.] 



KNOW^LEDGE 



179 



THE MAGIC WHEEL. 



ItT^E had lioped to prosent our jounger readers this 

 Vt week with a drawing of a trotting horse if ji.? 

 various positions successively assumed by the aniir . (as 

 instantaneously photographed), for use with th. —genious 

 instrument illustrated in the accompanying cut. But, 

 on carefully examining the picture in the Scientific 

 Americiin, we found that there was an error which 

 would have caused tlie picture to produce an imperfect 

 illustration of the horse's action. Twelve positions had 

 been taken from the photographer's series without its 

 being noticed that the last two were almost exact repe- 

 titions of the first two (in other words, a complete double 



step was illustrated in the first ten pictures). The delay 

 caused by the corrections prevents us from giving the 

 picture this week, but next week we shall have a set of 

 tea positions of a trotting horse, arranged for use as in the 

 accompanying figure, illustrating a highly ingenious 

 method for avoiding the difiiculties involved in the con- 

 struction of a zoetrope. We shall later give a series of 

 views of a galloping horse. In the meantime we leave 

 our younger readers to puzzle out the meaning of the 

 accompanying cut, and in particular to find out how it is 

 that tlie various parts of a properly-constructed zoetrope 

 are provided for here by so simple a construction. 



PRIMARY COLOURS. 



IT is impossible to construct a consistent theory based on three 

 primar)' colours, whether the three be the older set — bine, 

 yellow, and red, or violet, green, and red — the newer set of the 

 theory considered by some to have been established by Clerk 

 Maxwell in his paper to the Royal Society in 1860. 



Clerk Maxwell's paper contains serious errors. He forma equa- 

 tions with different kinds of quantities, implicitly attributing to the 

 units of those quantities values which, regarded relatively to each 



other, are purely accidental or arbitrary, and his results are, con- 

 sequently, fallacious. 



As is apparently recoscnised by Rood, in " Modern Chromatics, "- 

 a correct and complete theory of colour vision ought to enable us to 

 construct a circular diagram of colours, with complementary colours 

 diametrically opposite to each other, and with the colours distri- 

 buted with uniform gradation round the circle, and in accordance 

 with their true relations to each other. Now, such a diagram cannot 

 be constructed on the assumption that the primaries are three in 

 number, without assuming certain colours, or combinations of colour, 

 to be complementary, which are proved by actual experiment not 

 to be so. The assumption also involves the absurdity that a colour 

 can be (in a sense) complementary to itself ; or in other words, that 

 two diametrically opposite colours or combinations of colour which 

 are complementary.to each other may each contain the same colour 

 as an ingredient. 



When proceeding to arrange colours in a circular diagram, we 

 have first to classify them. For this purpose I take so-called 

 primaries and secondaries together, and for convenience call them 

 simply distinct colours. To the blue-yellow-red theorists I say that 

 to my eyes green and violet are as " distinct" as any of the three, 

 but orange is not to the violet-grcen-red theorists. I say blue 

 and yellow are as "distinct" as any of the three, but purple 

 or crimson is not. How many " distinct " colours, then, are 

 there to be assigned to equidistant points on the diagram ? In 

 my opinion the human mind cannot conceive of more than five 

 colours which are as distinct from each other as red from yellow 

 or from violet, or as blue from green or from violet, or as yellow 

 from green. If, then, there are only five colours of the first class, 

 or of the first and second classes together, it is impossible to con- 

 struct a theory with three primaries, for such a theory implies three 

 secondaries, or six colours of the first and second classes taken 

 together. 



^V'ith reference to seeing red in the violet of the solar spectrum, 

 I may mention the case of a person who sees the violet of the 

 spectrum as a dim grey only, and yet, when itwo spectra overlap, 

 so that the red end of one combines with the blue part of the 

 other, he apparently experiences the same sensations of violet and 

 purple as normal-eyed persons. 



It is held by Helraholtz and others, and I think there can be no 

 doubt of the fact, that mentally we cannot really distinguish in any 

 colour-sensation any components, but only a single resultant sensa- 

 tion. We may experience a sensation which may be called pure as 

 regards its colouredness ; for example, we may experience the 

 sensation of a green, which inclines neither to yellow nor to 

 blue, but it is quite certain that for a normal-eyed person 

 it is impossible for light to act on the eye in a perfectly 

 simple or pure manner. In this sense a perfectly pure colour 

 is not obtainable even from the solar spectrum itself, how- 

 ever much it be dispersed, or however narrow a portion of it be 

 t.akeu. To explain my meaning, I will suppose separate nerve 

 fibres of the retina are sensitive to the different primary colours, 

 whatever they may be ; then, what I assert, and can prove, is that 

 no part of the spectrum, however small, acts on a single fibre (ex- 

 cepting, perhaps, at and near the extreme ends of the spectrum). 

 Assuming this to be true, it follows that experiments, like those 

 recently described by Lord Rayleigh, in which the green and red 

 of the spectrum are combined and produce the sensation of yellow, 

 do not in the least prove that the sensation of yellow is neces- 

 sarily a compound sensation, or that yellow is not a primary colour. 

 A great deal of the difficulty arising in the consideration of the 

 effects of mixing colours disappears when it is understood that 

 colours neutralise rather than combine with or add to each other, 

 the resultant sensation being one which may be described as a mix- 

 ture of more or less white or uncoloured light, with as much of the 

 colouredness as is not neutralised. E. H. 



Glasgow, Dec. 3, 1881. 



Chari.es Brush, of Cleveland, Ohio, is declared to have perfected 

 a new invention for storing electricity. The design consists of a 

 battery in the same sense as in Plante's and Paure's, but the details 

 are entirely different, and do not infringe upon the rights of either. 

 Mr. Brush uses for his storage reservoir metal plates, so arranged 

 that they are capable of receiving a very large charge of electricity 

 and of holding it for an indefinite time. The storage reservoirs 

 van' in size as desired, may be transported from place to place, and 

 used as desired. Each citizen may then run his own electric light 

 as he pleases ; the plates can be put on street-cars, connected with 

 the axles, and made to run the cars without horses, .and steam-cars 

 mav be ultimately run in the same way. The practical character 

 of the invention is said to be settled, and it is simply a matter of 

 expense, but the details of the methods are not made public.^ 

 Frank Leslie's Magazine. 



