ABNORMALITIES IN THE REFRACTION OF THE EYE 



867 



The Indirect Method. — As the name indicates, indirect ophthalmoscopy con- 

 sists in the formation of a retinal image in space in front of the observer's eye, the 

 principle involved being similar to that of the compound microscope (Fig. 467). 

 The reflecting mirror is held at about an arm's length from the observed eye (30 

 cm.). A convex lens of about 20 diopters is then placed close to the, latter. Ob- 

 viously, the purpose of this lens is to gather the rays emerging from the observed 

 eye and to bring them to a focus between it and the observer's eye. This real 

 inverted image in space is regarded by the observer through a lens of about 5 

 diopters inserted in the orifice of the ophthalmoscope. To see this image clearly, 

 the emmetropic observer must move nearer to or farther away from the patient's 

 eye until his distance equals the focal distance of this lens, viz. : 20 cm. Errors in 

 refraction may be detected by moving the objective lens of 20 diopters farther 

 away from the eye, the image then becoming larger in myopia and smaller in hyper- 

 metropia. The observer then interposes different concave ( — ) and convex ( + ) 

 lenses until the image becomes perfectly clear. 



Fig. 467. — Indirect Ophthalmoscopy. 

 Diagram to illustrate the remittance of the rays of light by an emmetropic eye. 

 O, observer's eye; M, mirror; P, patient's eye; F, the rays FA and FB illuminate the 

 retina of P by a diffusion circle A^B^ (inverted in this case) L, the rays emitted by these 

 luminous points are converted into a real inverted image in the air at J. The latter is 

 then focused upon the observer's retina. 



Skiascopy or the Shadow Test (Retinoscopy). — This method con- 

 sists in determining the direction of the movement of the hght in the 

 pupillar orifice when it is made to move back and forth by rotating 

 the reflecting mirror around the long axis of the handle supporting 

 it. It is a matter of common observation that a beam of light reflected 

 against a wall, moves with the reflecting mirror. A similar phenome- 

 non occurs in the human eye if the retina is illuminated so that it 

 can emit hght. Thus, if a beam of hght is thrown into the eye, the 

 pupil is completely illuminated. If the mirror is now rotated around 

 its long axis, the pupil is darkened on one side and this shadow moves 

 either in the same or in the opposite direction to the rotation according 

 to the position of the observer's eye in the hne of vision of the observed 

 eye (Fig. 468). If situated exactly at its far point, the pupil remains 

 either dark or is fully illuminated and does not exhibit a distinct 

 moving shadow. This point indicates the position of the so-called 

 Voint oj reversal {A). Retinoscopy, therefore, is a method by means of 

 which the distance of this point may be accurately determined. Be- 



