An additional method to determine the degree of ametropia.

  • William Thomson
Date:
[1871]
Catalogue details

Licence: Public Domain Mark

Credit: An additional method to determine the degree of ametropia. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    5/6 page 7
    Having satisfied ourselves that an ametropia does exist in any given case, and that the visual axis is too long or too short, we can measure the axis with precision by using the Listing eye for comparison. We know that when an object is placed so near the eye as to give divergent instead of parallel rays, its image must fall behind the position of the posterior focus. These points of emission and convergence are the conjugate foci, and are found by the formula f'f = F' F] f being the distance of the object from the anterior focus, and / the dis- tance of the image from the posterior focus; F' and F being the known quantities of the anterior and posterior foci. The proportion is /': F::F : /; hence f = JP1 jp'i ~Y~ In the case of Mrs. G., we found a myopia of }.5, and we know that an object distant 6J inches from her nodal point has its image on her retina. Should we desire to ascertain how much her visual axis is too long, assuming with the best autho- rities that it should be normally 22.28 mm., we subtract from 6.5 = 165 mm. the distance from nodal point to anterior fo- ’fill Tjlji cus = 20 mm., and we have f =145; to find f = £ £_ we f 20x15 J say / = = 2.07. The retina is 2.07 mm. distant from J 17 145 its proper position, and her axis is therefore 22.28 + 2.07 = 24.80 mm. in length. The same formula is used for hypermetropia; but as the dis- tance from the nodal point to the object is negative, f is found by adding the distance of the point of convergence behind the eye to the distance from the nodal point to F'\ and with hy- permetropia of 4.5 = 165 mm. f — 165 + 20 = 185,/ = 20 x 15 •= 1.6. Subtract this quantity from 22.28, and we have 185 4 ■ the length of axis which gives rise to this degree of hyperme- tropia = 20.63. An instrument for making these examinations has been con- structed for me by Mr. Zentmayer, which consists of four disks of sheet-brass 1J inches in diameter, attached together by a pivot passing through a small projecting handle upon each disk.
    digitised image 1
    digitised image 2
    digitised image 3