26 THE OPTICAL DEFECTS OF THE EYE. 



permanently, his accomodation must first be gradually relaxed by the 

 use of weaker lenses. Donders' rule is to prescribe first that glass 

 "that will neutralize his manifest hypermetropia, and \ of his latent 

 hypermetropia, and every two or three weeks change them for a 

 ■stronger pair, as he becomes accustomed to their use, until the glasses 

 are reached that we found to be necessary to correct his hypeimetro- 

 pia. Thus, if a patient has a total amount of hypermetropia equal 

 to r^, and a manifest hypermetropia of ^^, his latent hypermetropia 

 (t& -• To = tV)> would equal ^V ; one fourth of Jg- is -g-V ; this, 

 added to -g\ (^ + ^ = _3_ = _i_), equals ^. We would therefore 

 prescribe, at first, 20 inch convex spectacles, which we would after- 

 wards change successively for + 18, + 16, + 14, &c., until he has so 

 relaxed his accomodation that he can, with ease, wear 4- 10. It 

 will not be until he becomes accustomed to this last pair that all his 

 symptoms of Asthenopia will disappear. 



Strabismus. — Prof Donders was the first to direct attention to the 

 fact, that nearly all cases of convergent strabismus arise from the 

 presence of hypermetropia, "We know that when both eyes are 

 directed to a near object, they are very much converged, — the optic 

 axes cross at the point to which they are] directed. If one eye be 

 covered, and the opposite eye be accommodated for its " near " point, 

 the convered eye will be found to be very decidedly converged towards 

 the nose, — to have, in fact, a temporary convergent s^^uint. This 

 arises from the constant association of the act of accommodating the 

 eye for short distances, with the act of contracting the internal recti 

 muscles. The hypermetropic, however, being obliged to exert the 

 accommodation of their eyes, even when looking at distant objects, it 

 is easy to understand that they would be inclined to contract their 

 internal recti-muscles unduly, so as to increase this power of accom- 

 modation. This converges the eyes to a point at a nearer distance 

 than the object looked at, and causes one ot the eyes to turn inwards, 

 while the other is fixed upon the object. When, therefore, they wish 

 to see distinctly with one eye, they instinctively turn in the other. 

 At first the convergent strabismus is seen occasionally only, and in 

 this stage may be prevented by using the proper spectacles to correct 

 the hypermetrophia. After the squint has existed sometime, it 

 becomes confirmed and cannot be cured without an operation. 



If the convergence exceeds three lines, a partial tenotomy, upon 

 •each eye, should be performed, and the effect controlled by a conjunc- 



