Cli-1 Mp. Ernest Clarke [June 9, 



from the axis, the maximum refraction Ijeing- at right angles to the 

 axis. A convex cyHndrical lens is made, as it were, from a solid 

 cylinder of glass, and the concave cylinder may be considered to be 

 the mould of the convex cylinder. 



The vision of an astigmatic person who has a gross error is not 

 like the vision of a myope. For instance, the myope might not see 

 even -^-^^ but a person suffering from gross astigmatism might see 

 even some letters in the normal line, but in a " fuzzy " manner. [A 

 demonstration was then given of the vision of an astigmatic person 

 and the effect of correcting it by a cylinder.] If the cylinder is 

 turned a little out of the true the picture at once becomes indistinct,, 

 hence the importance of making sure that the axis of the cylinder 

 witli which you are correcting the astigmatism is absolutely true.. 

 These gross errors are very simple ; they take care of themselves, and 

 go to the oculist or the optician to be corrected. 



But the real importance of this subject is only revealed when we 

 come to discuss the effect of small errors. The possessor of this- 

 form of astigmatism is usually quite unconscious of it, and when an 

 up-to-date physician suggests that the headaches or neuralgia, or 

 some other functional neurosis, is due to the eyes, they are often 

 indignant, declaring the vision to be perfect, which apparently it is. 

 The fact is the ciliary muscle is correcting the defect, if, for instance, 

 the horizontal curve of the cornea is flatter than the vertical, the 

 ciUary muscle can contract irregularly and cause the vertical curve of 

 the lens to become flatter and so correct the astigmatism. It is only 

 very small errors that can be corrected in this way. The individual 

 is not conscious of this work done, hit the nervous system is. 



Associated with this is another cause of eyestrain, viz. anisome- 

 tropia, " odd " sight. That, as you see in the diagram, is much 

 more prevalent than people would imagine. Out of these 2500 

 individuals no less than 1843 had "odd" sight. If the amount of 

 difference between the two eyes is very small, i.e. 0'25D or 0*5 D, 

 impulses can pass down from the brain to the ciliary muscle, and 

 the anisometropia is unconsciously coiTected. 



The third cause of eyestrain is muscle- strain. AVe who have 

 perfect sight, and see only one object when we look at it, are 

 unconscious of the fact that we really see two, and that by the 

 wonderful balance of our muscles those two pictures are merged 

 together to form one. If there is a small want of balance between 

 the external muscles of the eye, impulses pass down from the brain 

 to one muscle, and the balance is preserved and diplopia prevented, 

 but at the expense of nerve energy. 



Those three forms of eyestrain may co-exist, and you can easily 

 imagine the enormous waste of nerve energy going on all the waking 

 hours, caused by the constant passage of nerve impulses to the 

 muscles from the brain. How insidious this is we reahze when we 

 know that sometimes there is no symptom to point the danger. 



