:n 



Si.U'lXTlNi!. 



8TAK1.K. 



711 



The rule for extracting the square root of an algebraical quantity is 

 very little needed, but if a remarkably good exercise in the operationa 

 of algebra. Arranging the square in powen of some one letter, the 

 rule u identical with that for the square root of numbers in every 

 point but this, that the new term is always found by dividing the tint 

 Irrm of the remainder by the .*>< term of the divisor. A couple of 

 examples will suffice. Let it be required to find the square root of 



Zx? + 2** + 'Ac* + a* 



Again, to find the square root of 1 +.r : 



16 ~ 128 + ' ' 



2 + *-T I-J- 

 a? 



T 



- 



8 



z< 

 64 



2 + X , -t-Tflla itl 



at 



64 



16 



^ 



64 



256 



64 



f! 

 64 



_^ 

 256 



SQUINTING (Stroliiimut). It is a condition essential to correct 

 vision that the axes of both eyes correspond in direction, and be 

 turned simultaneously towards the object we regard. Now to ensure 

 the fulfilment of this condition, the orbital muscles (motores oculorum) 

 are so supplied with nervous influence, that we cannot will the move- 

 ment of one eye without the other being called into involuntary and 

 harmonious action. There are some individuals, however, whose optic 

 axes are not parallel, and whose eyes do not move in harmony with 

 each other ; such persons are said to squint, or to be affected with 

 strabismus. Squinting may take place either upwards, downwards, 

 inwards, or outwards, or in the intermediate directions ; it may also be 

 Confined to one eye ; or may affect both. Aa the inward and outward 

 varieties of squint are by far the most common, we shall devote the 

 following remarks to them alone. 



Inward Squint, or Strabitmus convergent, is met with in three distinct 

 forms : 1, single convergent strabismus ; 2, double convergent strabis- 

 mus ; and 3, alternating strabismus. In the first form of the affection 

 one eye is habitually turned more or less inwards towards the inner 

 angle of the orbit, whilst the other maintains its natural petition, 

 and is capable of being directed to any object that the individual 

 \\.~lf -. 



On closing the sound eye, the inverted one then becomes straight, 

 and can be turned in every direction nearly to the same extent as the 

 other ; but as soon as it is again opened, the one affected with stra- 

 bismus revolves inwards, and there remains ; or if it do move along 

 with the good eye, yet never so as to permit the two axes to be pointed 

 at the same object. Double convergent strabismus differs from the 

 preceding in its affecting both eyes ; the axis of each eye is inclined 

 unnaturally inwards, as if the person were regarding some object 

 placed close to his face. On directing his attention to distant objects, 

 the eyes do not become parallel, but the one least affected (for one is 

 always more so than the other) becomes straight, whilst its fellow pre- 

 serves its former position, or is turned more strongly inwards. With 

 regard to the relative frequency of strabismus in one or in both 

 eyes, it is said to affect most frequently one eye only, and this 

 the left. 



Alternating Strabismus differs from the ordinary form of squint, in 

 its affecting both eyes equally, though never both at a time. An in- 

 dividual thus affected appears to use either eye indifferently ; and the 

 change of inversion from one eye to the other is a voluntary act, 

 independent of the opening or closing of the eyelids. 



Ou'vard Squint, vr StrMtmtw divergent. Nearly all that has been 



said in rofi-ri-ni-c to convergent strabismus, may be applied, mtilntii 

 mutandit, to divergent squint. In thin form of the affection, one eye 

 is more or loss everted, whilst the other is directed straight forwards ; 

 the patient is likewise incapable of directing both eyes inwards simul- 

 taneously. These cases we believe to be most frequently of the 

 alternating kind ; that is, the individual can employ either eye singly, 

 and bring it into the central axis, but then its fellow becomes everted. 

 It is a more rare affection than the former one, and the deformity 

 arising from it is seldom so obvious. Whether we regard strabismus 

 as affecting one eye or both, it is certain that the vision of the one 

 most distorted is nearly always imperfect, and usually in a direct ratio 

 with the degree of distortion. Now we know that if impressions on 

 the two retinae are dissimilar in force, the mind disregards the weaker, 

 and takes cognizance only of the stronger ; BO that a person who 

 squints badly generally sees objects with the sound eye only. If the 

 sight of both eyes is equal or nearly so, double vision results when- 

 ever both are employed together, because the images of objects do not 

 fall on corresponding portions of the two retime [SIGHT, DEFECTS OF] ; 

 and as the defect of sight is generally in a direct ratio with the degree 

 of distortion, double vision is most frequently experienced in slight 

 cases of squint. 



Cautet. The inequality of power in the two eyes has been regarded 

 by many as a cause of strabismus ; the defective eye, it is said, " in- 

 stead of being fixed on the object before it, is left to wander from the 

 true axis of vision." When, however, we consider how numerous are 

 the examples of unequal vision with the two eyes, yet unattended 

 with squint, and the great and immediate improvement of sight which 

 generally results from the operation for the removal of the defect, we 

 may fairly question the influence of this cause in the production of 

 strabismus. Among the remote causes which unquestionably con- 

 tribute to this effect, may be enumerated convulsions, teething, the 

 irritation arising from worms, ophthalmia, imitation, a habit of mis- 

 directing the eyes, as by frequently looking at a mole on the nose, &c. 

 The proximate cause resides in some affection of the muscles or nerves 

 of the eyeball; either the balance of power between the former U 

 lost, or the sympathy which exists naturally between the motor-oculi 

 nerves of the two eyes is impaired. 



Treatment. This must depend upon whether the affection is of a 

 temporary or permanent nature ; in the former case it will be found 

 to arise from some local irritation, and can be removed by suitable 

 therapeutic remedies; in the latter, an operation will generally be 

 required. Among the different other plans of treatment which occa- 

 sionally have proved successful, we may enumerate binding up the 

 sound eye ; the employment of spectacles having glasses of different 

 power; blinders projecting in front of the temples, with a view of 

 attracting the eyes outwards ; electricity, &c. The operation for the 

 cure of strabismus is said to have suggested itself first to Dr. Stromeyer, 

 from witnessing the success of tenotomy in contractions of the limbs. 

 Dr. Dieffenbach of Berlin, however, was the first who had the boldness 

 to carry it into practice on the living subject. The operation consists 

 in dividing the muscle by which the distortion is produced, and thus 

 allowing its antagonist to draw the eye again into the centre of the 

 orbit. Although most cases of strabismus may be either completely 

 cured or very much bettered by this operation, it is proper to remark 

 that in some, neither this nor any other plan of treatment is of any 

 avail. Provided, however, that the subjects to be operated on are 

 judiciously selected, and the surgeon qualified for the task, there is no 

 operation within the whole range of surgery which is more simple, 

 more free from danger, or more satisfactory in its results, than the one 

 in question. 



STABLE AND UNSTABLE ; STABILITY. A system is said to 

 be stable when a slight disturbance of its actual condition would not 

 produce a continually increasing effect, but one which finally ceases to 

 increase, diminishes, becomes an effect of a contrary character, and so 

 on, in an oscillatory manner. The ordinary vibration of a pendulum 

 is an instance ; the oscillation takes place about a stable position of 

 equilibrium. We can give no instance of an unstable position ; for by 

 definition, such a thing is a mathematical fiction. Any disturbance, 

 however slight, produces upon an unstable system an effect which con- 

 tinually increases : no unstable equilibrium therefore can exist a 

 moment, for no system made by human hands can be placed with 

 mathematical exactness in a given position. The pendulum has a 

 position of equilibrium exactly opposite to that about which it 

 can oscillate, but no nicety of adjustment will retain it in that 

 position : it may appear to rest for a moment, but will almost instantly 

 begin to fall. 



The following curves or lines are all such that, supposing them to 

 be rigid matter, a molecule placed at A would rest : 



In the first, a displacement to the right or left would produce 

 nothing but oscillation, and the equilibrum is stable ; in the second, 

 neither displacement would be followed by any tendency to restore- 



