SIGHT, DEFECTS OF. 



SIGN. 



563 



condition of that organ. The glasses that are most commonly used 

 are double concaves, of equal concavity on each side ; they are num- 

 bered 1, 2, 3, 4, &c., beginning with the longest focus or shallowest 

 concavity. Unfortunately there is no uniform standard adopted in the 

 manufacture of these glasses, so that what one optician calls No. 1, 

 another rates as No. 2, and so on ; it is therefore advisable that those 

 who wish to fit themselves with spectacles should try a series of them 

 at an optician's shop, and they should be content with the lowest 

 number with which they can see objects clearly across the street ; if 

 it diminishes them much, or gives them a dazzling appearance, or if 

 the eyes feel strained after looking through them for a short time, 

 they are too concave. Spectacles are always preferable to a single eye- 

 glass ; and when the individual has met with a pair which suit him, 

 they should not be heedlessly changed for any of deeper concavity. It 

 is also advisable not to wear them constantly, but only on occasions 

 when their assistance is absolutely required. 



Presbyopia, or Far-sightedness (from vpeafivs, " old," and tty, " eye ;" 

 this being a state of vision to which old age is almost invariably 

 subject.) 



Long-sightedness, as the name sufficiently indicates, is an affection 

 the reverse of the one just described, and depends upon opposite 

 causes. Either the refractive powers of the eye are too feeble, or its 

 axis is shorter than is natural ; the result is an imperfectly formed 

 image on the retina, from the rays of light not converging sufficiently 

 soon to be brought to a focus. Hence the far-sighted person removes 

 the object he is examining farther from him, or he makes use of 

 glasses whose effect is to increase the refraction of the rays of light 

 before they enter his eye. Far-sightedness is sometimes met with in 

 the young ; bat it is rare that an individual lives to be old without be- 

 coming presbyopic : indeed the sinking of the eyeballs, the flattening 

 of the cornea; , and the smallness of the pupils, all which contribute to 

 this effect, are among the series of changes which every part of our 

 body undergoes as we advance in age. The time of life at which pres- 

 ; first shows itself is generally about forty-five ; but there are 

 great differences in this respect, some persons requiring the use of 

 couvex glasses at thirty as much as others at fifty. Among the earliest 

 symptoms observed, are a difficulty of reading small print, in nibbing 

 a pen, or in examining small objects ; the letters of a book appear 

 misty, and run one into another ; and if the effort is long continued, 

 the eyes become fatigued and the head aches. Notwithstanding this 

 difficulty of distinguishing near objects, distant ones continue to be 

 seen as clearly as before. 



Presbyopia, after it has once appeared, generally goes on increasing, 

 so that an individual thus affected requires to change his glasses from 

 time to time for those of a higher power : instances, however, are 

 recorded of old persons long accustomed to the use of convex glasses 

 recovering their former sight at the age of 80 or 90 years. 



Treatment. The same principles that we have laid down for the 

 treatment of myopia, and for the use of concave glasses, are applicable, 

 regard being had to the opposite condition of the eye, to the present 

 affection. Convex glasses should not be had recourse to too soon, nor 

 should too high a power be used, but the lowest that answers the 

 purpose is to be chosen. When presbyopia occurs suddenly, and in an 

 individual much below the age at which it ordinarily occurs, there is 

 some mischief to be suspected either in the eye or in the brain, which 

 will require an antiphlogistic treatment and a total suspension of the 

 use of the eyes in regarding near or small objects. 



Double Vuion, Vital duplicatui, or Diplopia, may arise either from 

 a want of correspondence in the movements or position of the two 

 eye, the vision of each eye singly being perfect ; or there may be 

 double vision with one eye only, while the harmony hi the movements 

 i if the two is not disturbed. The most common example of the first 

 form of the affection is afforded by cases of squinting ; but as this 

 defect is treated of in a separate article, we merely allude to it in this 

 place. More serious and less common is the loss of harmony in the 

 movements of the eyes which results from paralysis of one or more of 

 the orbital muscles. If one muscle only is affected, the eye will move 

 in harmony with its fellow in every direction but the one towards 

 which its paralysed muscle should draw it, consequently in this direc- 

 tion objects will be seen double ; but if several are affected, as is not 

 unfrequently the case, then the^movementa of the eye will be still more 

 restricted, and there will be single vision only when the axis of the 

 sound eye is parallel with that of the paralysed one. These affections 

 appear sometimes to arise from cold ; at other times they arc depen- 

 dent upon disease about the base of the brain, as some tumour pressing 

 on the motor oculi nerve, or there may be an inflammatory condition 

 of the brain and its membranes, or a sanguineous or serous effusion 

 involving the origin of the third pair of nerves. Whichever of these 

 may be the cause, our treatment must be directed to remove it, while 

 the state of the eye will be an index of the success or failure of the 

 remedies we make use of. 



Double vision with a single eye is a more rare affection than the one 

 just described, and depends upon some irregular refraction of the 

 cornea or lens. 



M. PnSvost, who published an account of his own case in the ' An- 

 nales de Cliiinic et de I'hyxiijue,' 1832, thought it might arise from a 

 fracture, bruise, or partial flattening of the lens, or separation of its 

 lamina:. Professor Airy ami Mr. Babhage are troubled with this 



ARTS ASD SCI. DIV. VOL. VII. 



defect, the latter gentleman with both eyes, but he is able to remedy 

 it by looking through a small hole in a card, or through a concave lens. 

 Professor Airy finds that his eye refracts the rays to a nearer focus in 

 the vertical than in the horizontal plane, and he has ingeniously con- 

 trived to remedy it by the use of a double concave lens, one surface of 

 which is spherical and the other cylindrical. The spherical surface is 

 to correct the general defect of a too convex cornea ; the cylindrical 

 is to converge or diverge those rays at right angles to the axis, while 

 the parallelism of those which impinge upon it in the plane of its axis 

 is unaffected. Thus the focus of the spherical surface will remain 

 unaltered in one plane, but in the other it will be changed to that of a 

 lens formed by it and a spherical surface of equal curvature with the 

 cylinder. With the aid of a glass of this description Professor Airy 

 could read the smallest print at a considerable distance equally well 

 with the defective as with the sound eye. He found that vision was 

 most distinct when the glass was pretty close to the eye and the 

 cylindrical surface turned from it. " With these precautions," he 

 observes, " I find that the eye which I once feared would become 

 quite useless, can be used in almost every respect as well as the 

 other." 



Colour Blindness ; Dichromism ; Chromatopseudopsls ; Defect of the 

 Sense of Colour ; Daltiminm. There are some persons who, although 

 they can see the size and form of objects perfectly well, have neverthe- 

 less a deficient power of distinguishing colour. Many distinguished 

 individuals have been subject to this : such as Dugald Stewart, M. 

 Sismondi, and John Dalton, the celebrated chemist. The subject 

 first excited general attention by the latter distinguished individual 

 having reported his own case, and the deficiency was hence called 

 Daltonism. This subject was investigated by the late Dr. George 

 Wilson, of Edinburgh, who has written a work on ' Colour Blindness ' 

 (1855), which contains by far the fullest account of this defect which 

 exists. Dr. Wilson divides the colour- blind into three classes. 1. 

 Inability to discern any colour properly so-called, so that black and 

 white, that is, light and shade, are the only variations of tint perceived. 

 2. Inability to discriminate between the nicer shades of the more com- 

 posite colours, such as browns, grays, and neutral tints. 3. Inability 

 to distinguish between the primary colours red, blue, and yellow ; or 

 between those and the secondary colours, such as green, purple, orange, 

 brown. The last variety is the most common. In all these cases it 

 would appear that the primary defect is the inability to distinguish the 

 red colour, and the influence it exerts on the other colours of the 

 spectrum. Dr. Wilson has narrated a large number of cases, and shown 

 that this derangement of vision is much more common than has 

 hitherto been supposed. He calculates from his own observations that 

 at least one person in fifty is thus affected. Dr. Wilson has shown 

 that it becomes very important to know where this defect exists, as it 

 incapacitates people for occupations where the appreciation of colour is 

 important. He has specially shown that, in cases of engine-drivers 

 on railways and sailors at sea, who are warned of danger by coloured 

 signals, this condition may lead to serious disasters. 



The cause of this phenomenon has occasioned much discussion. 

 Daltou attributed it to the condition of. the humours of the eye ; 

 but a careful examination of his eyes after death, by Mr. Kansome, 

 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 ' On 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 

 adopted Young's theory that there is a distinct retinal structure for 

 the perception of each colour, and shown 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 coloui- 

 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.,' 1851-5, vol. xxi. part ii.) 



SIGN (Astronomy), a constellation; but in modern times a con- 

 stellation of the ZODIAC only. For the distinction of the sidereal and 

 astronomical zodiac, see PRECESSION. 



SIGN (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 iu 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 ALGF.BHA, ax 



o o 



