T15AXSACT10XS OV SECTION A. 



463 



5. Improved Apparatus for the Objective Estimation of Astigmatism. 

 Bij Tempest Andeeson, M.I)., B.Sc. 



Astigmatism lias been defined as that condition of tlie eye in which refraction 

 is unequal in the different meridians. In order to obtain suitable spectacles for 

 corrcL-tiug: this defect, it is necessary to huow accurately the focal adjustment of 

 the meridians of maximum and minimum curvature, whence the focal lengths of 

 glasses, generally either cylindrical or cylindrical on one side and spherical on the 

 other, are readily calculated. Many plans have been adopted for determining this ; 

 some subjective, depending on observations made by the eye itself, and generally 

 using a point of light or a series of radiating lines as an object. From their 

 appearances -when viewed at different distances, and with lenses of different powers, 

 the focal adjustment of the different meridians is at last obtained. 



Plan. II. 



References to both figures : — 



A. Observed eye. 



B. Observing eye. 

 m. Mirror. 



/. Lamp. 



c. Condensing lens. 



w. Wire screen, seen edgeways. 



y. Principal lens. 



X. Correcting lens. 

 Plan I. — C. Graduated bar. 

 Plan II.— Z). Tube. 



The advantage of this group of methods is their theoretical delicac}-, as they 

 work by judging of the perfection of certain images refracted on the retina in a 

 manner not very dissimilar to that in which they are usually formed ; the jn-ac- 

 tical disadvantage, that accurate observations are required from one who has never 

 been accustomed to make them. Hence objective methods have been introduced. 

 Their advantages are, substituting trained for untrained observation. Their 

 disadvantages — (1) The vessels of the retina and the optic nerves, which are 

 mostly employed as objects, are seldom iu exactly the position desirable for 

 estimating the refraction in different meridians, and are often at a diflerent distance 

 from the optical system of the eye from that at which the sensitive layer of the 

 retina lies. 



(2) They mostly require the optical defects, if any, and the accouimodatiou of 

 the observing eye to be taken into account and allowed for, thus introducing risk 

 of error. 



In the author's two instruments, an image of a suitable object thrown on the 

 retina of the observed eye, is used as an object b}- the observer, with the following 

 advantages : — 



