The cause of this phenomenon has occasioned much discussion. Dalton attributed it to the condition of. the humours of the eye ; but a careful examination of his eyes after death, by Mr. Rausome, revealed nothing to account for the defect. It has been supposed by others that the defect is connected with the organisation of the brain. Recently Professor Clark Maxwell, of Cambridge, who has written a paper the Compound Theory of Colours,' in the Philosophical Transactions' for 1860, explaining his views, has proposed another theory, which has been generally accepted. He has shown that the three primitive colours are not, as usually regarded, red, yellow, and blue, but red, green, and blue, as pointed out by Young. He has also Young's theory that there is a distinct retinal structure for the perception of each colour, and showu how the remarkable defect of appreciation of the red rays occurs in the colour-blind. Professor Maxwell has also suggested the only means of alleviating this defect which has hitherto been recommended, and that is the wearing of spectacles composed of red and green glass simultaneously. A spectacle frame of the usual kind is constructed with one glass red, the other green, so that the right eye, for example, of the wearer of the spectacles looks always through red and the left always through green. Through the red glass red objects appear brighter than green ones, through the green glass green objects appear brighter than red ones, so that a coloms blind person puzzled between red and green has only to determine whether the doubtful colour appears brighter to the right or the left eye, and to set it down as the colour of the glass which brightens it. (See Maxwell On Colour as perceived by the Eye, with remarks on Colour-Blindness," Trans. Roy. Soc. Edin., 1854-5, vol. xxi. part ii.) SIGN (Astronomy), a constellation; • but in modern times a con stellation of the ZODIAO only. For the distinction of the sidereal and astronomical zodiac, see PRECESSION.
(Mathematics). Every symbol is a sign of something or other, the original meaning of the word applying to any mark of distinction or designation. The general consideration of the subject of signs comes under the word SYMBOL ; for this term, sign, is exclusively applied in mathematical analysis to the signs of addition and sub traction ( + and —). A positive quantity, as +3, is said to have the positive sign ; a negative quantity, as —3, the negative sign.
The theory of these signs is the peculiar feature of Aionsen, as distinguished from arithmetic; and It is difficult to place it on any satisfactory bash except that of distinct definitions not wholly derived from arithmetic. On this point, however, it is not our present purpose to enter further ; the object of this article being the application of the signs, and in particular Howl details of interpretation which are neces sary In the application of ordiusry algebra- to geometry. By ordinary algebra we mean that system in which the positive and negative quantities are fully capable of interpretation, but in which s/ -1 is considered as incapable of interpretation.
The relative meaning of + and - is direct opposition of properties; and it is only where direct opposition is possible that complete inter pretation can exist. The symbol + 7 means not only 7 units of its
kind, but 7 units directed to be considered in a specific one of two (the only possible) lights, or used in a specific one of two (the only possible) manners ; the first generally implying the second. Thus let 7 inches be measured from a given point; the superposition of + or - tells nothing, for the measurement rosy be made in an infinite number of different directions. Choose one of these directions, or rather one line of direction, and the indeterminate character of the proposal (to measure seven inches from the given point) is almost gone there are but two directions in which to do it ; if one of them (no matter which) be signified by + 7, the other (no matter which, except with reference to the first) must be denoted by -7.
A problem may present different sets of oppositions of very different kinds. Thus we might have a problem in which there are concerned together-I, time before or time after a certain epoch ; 2,beight above or height below a certain level ; 3, the debtor or creditor side of certain books. To give a more precise idea (it is hardly worth while to frame a specific problem), a man might engage to build a wall on different ternie as to the foundation and what is above the ground ; for which he might have to borrow money and pay interest up to a certain time, when by receiving the whole amount due to him he might repay and invest besides; and the whole transaction might have to be properly entered in his books. The young student might suppose that if + I and -1 represent a foot of the wall above and a foot below the ground, it will not necessarily follow that + 1 and - 1 (undistinguishable from the former) will do to represent one pound of Interest due to and from the contractor ; and still less that the same + 1 and - I will also do to direct 1L to be carried to the debtor or creditor side of his books. But what he will learn from a properly established algebra (and until he has learnt it, he is not in possession of any part of the difference between algebra and arithmetic) is this-that he would gain absolutely nothing by inventing such distinctive symbols as would remove his doubt of their applicability. Let (+ )1 and ( -) I represent feet of wall, [+11 and [-] 1 pounds sterling of interest, + 1 and 4.- } 1 pounds sterling carried to one side or the other of the books; while + 1 and - I represent simple addition or subtraction. Let the problem be fairly translated into algebraical language, and an equation formed in which all the distinctive symbols are seen : algebra teaches that the rules to be applied to that equation differ in no respect whatever from those which would have been applicable if all the signs had come from the same source of meaning. Perhaps it would be better if the student were not allowed to come so easily by this result as he usually does, but should be made to learn by his trouble how unnecessary the dis tinctions really are, as to operations, and allowed in due time to feel the relief afforded by dropping them.