DIOPTKIC MECHANISM OF THE EYE 329 



surface is twice the distance of that between the lamp and the 

 mirror. 



The images are sharp when the image of the flame falls on 

 the retina of the observed eye, if this is accommodated for 

 distance. When the observer is 1 metre away from the observed 

 eye and the lamp, the virtual image of the latter will be 2 metres 

 from the eye under observation. If the subject can see the 

 image plainly, there must be not more than 0-5 D. of myopia, 

 otherwise the retinal image is more or less blurred, in proportion 

 with the degree of ametropia. 



When at the above distance a hypermetropic, emmetropic or 

 slightly myopic (myopic by less than 1 D.) eye is examined, a 

 shadow is seen in the pupil which, on rotation of the mirror, is 

 displaced in the same direction, and vertical to the axis of 

 rotation of the mirror. When, on the contrary, the observed eye 

 is myopic to more than 1 D., and its distance point consequently 

 lies nearer than the observer's eye, then the displacement of the 

 shadow takes place in the opposite direction to the movement of 

 the mirror, owing to the crossing of the rays that converge from 

 the myopic eye to its far point. The first is known as the.dfo'ratf, 

 the second as the indirect shadow. 



To determine the degree of ametropia it is now only necessary 

 to place a plus or minus correction lens in front of the observed eye, 

 so as to transform the direct into an indirect shadow, or vice versa. 

 The strength of this correction lens gives the degree of the myopia 

 or hypermetropia. During examination, the accommodation of 

 the observed eye must be absolutely relaxed ; the observer's eye, 

 on the contrary, must be accommodated to the plane of the pupil 

 of the observed eye. The details of this method will be found in 

 all recent text-books of ophthalmology. 



BIBLIOGRAPHY 



The most important general treatises of Physiological Optics, which cover the 



subjects dealt with in this chapter, with full references to ancient and modern 



authors, are : 



A. FICK. Gesichtssinn. Hermann's Handb. der Physiologic, iii. Leipzig, 1879. 



HELMHOLTZ. Handbuch der physiologischen Optik. 2nd ed. Hamburg and 

 Leipzig, 1886-1896. 



TsGHEBNING. Optique physiologique. Paris, 1898. 



C. HESS. Refraktion und Akkommodation des menschlichen Auges. Grafe-Samisch's 

 Handbuch der Augenheilkunde, viii. Leipzig, 1902-1903. 



A. BROCA, E. JAVAL, SULZER, TSCHERNING. Optique geometrique, Ophthalmo- 

 metrie, Dioptrique oculaire, Ametropies focales, Astigmie. Encyclopedic 

 francaise d'Ophthalmologie. Paris, 1904. 



F. SCHENCK. Dioptrik und Akkommodation des Auges. Nagel's Handb. der Physio- 

 logic des Menschen, iii. Brunswick, 1905. 



The more recent work is described in : 

 W. EINTHOVEN. Die Akkommodation des menschlichen Auges. Ergebnisse d. 



Physiologic, i. Part ii., 1902. 



H. SNELLEN, JR. Uber die Skiaskopie. Ib. iii. Part ii., 1904. 

 V. ROHR. Zur Dioptrik des Auges. Ib. viii., 1909. 



