THE OPTICAL DEFECTS OF THE EYE. 



2§ 



as to render the person hypernjetropic ; this form of hypermetropia 

 seldom exceeds -^-^. When a person has both hypermetropia and 

 prebyopia, it is necessary for him to use a stronger pair of glasses for 

 reading, &c., than for ordinary use. If a person for instance, wears 

 a pair of 1 8 inch convex spectacles to correct a hypermetropia of ■^-^, 

 and as age advances his "near" point recedes to 12 inches, even 

 with the addition of his glasses, it will be necessary for him to wear, 

 for reading, a pair of glasses having a focus of about 10|^ inches. 

 Thus i — ^2" = J^ ••= presbyopia, this added to the lens to correct 

 his hypermetropia, (^V + a'* = xs^i nearly) equals 10^ nearly. 



In the very aged, it is necessary to prescribe glasses, that will 

 enable them to read at 5 or 7 inches from the eye, as their vision is 

 usually somewhat impaired. 



The following table constructed by Dr. Kitchener may give a 

 general idea of the glasses required at diflPerent periods of life when 

 the presbyopia is unaccompanied by hypermetropia or amblyopia. 



At 40 years, — 36 inch focus. 



At 70 years, — 12 inch focus. 



Prof. Bonders thinks that when there is no hypermetropia present 

 we should generally advise those glasses to be worn that will enable 

 the person to read distinctly No. I (smallest) test type at a distance 

 of 12 inches. 



There is an optical defect of the eye that is occasionally met with 

 called astigmatism (from a and a-Ttyixa) in which horizontal and verti- 

 cal lines are not brought to a focus at the same distance behind the 

 crystaline lens. It is relieved by glasses specially ground for each 

 case, these glasses are cylindrical. I have seen but one case of 

 astigmatism. 



A very comprehensive article on this subject appears in the Medical 

 Times and Gazette, Nov., 1864, from the pen of J. Zachariah Laurence, 

 M.B., of London. 



The paralysis of the accommodation of the eye I have already 

 referred to in a case on page 14. 



