RETINOSCOPY. 



767 



while at the same time is indicated at a certain aperture the lens presented at the sight hole. 

 The instrument is also provided with a movable arrangement carrying a concave mirror at 

 either end. One of these mirrors is 10 inches in focus, and is used for indirect examination 

 and retinoscopy, while the other is of 3 inches focus for direct examination, and is fixed at an 

 angle of 25.] 



[Retinoscopy. The ophthalmoscope is used also for this purpose. A beam of light is 

 reflected into the eye by the ophthalmoscopic mirror, and the play of light and shade on the 

 fundus oculi observed. A study of this is important in determining anomalies of refraction. 

 For the method, the student is referred to a text-book on "Diseases of the Eye."] 



[Artificial Eye. The student may practise the use of the ophthalmoscope on an artificial 

 eye, such as that of Frost (fig. 554) or Perrin.] 



Illumination. In order to illuminate the interior of the eye, v. Helmholtz used several 

 plates of glass, placed behind each other, in the position of S, S, in fig. 548. Afterwards he 

 used a plane or concave mirror of 7 inches focus (fig. 549), with a hole in the centre. Fig. 552 

 shows the appearance of the fundus of the eye, as seen with the ophthalmoscope. In albinos 

 the fundus of the eye appears red, because light passes into the eye through the sclerotic and 



Fig. 555. 

 Action of the orthoscope. 



Fig. 554. 

 Frost's artificial eye. 



uvea, which are devoid of pigment. If a diaphragm be placed over the eye, so that the pupil 

 alone is free, the eye appears black (Bonders). 



Tapetum. In many animals the eyes have a bright green lustre. These eyes have a special 

 layer, the tapetum, or the membrana versicolor of Fielding : in carnivora it consists of cells, 

 in herbivora of fibres, placed between the capillaries of the choroid and the stroma of the uvea. 

 These structures exhibit interference-colours and reflect much light, so that the coloured lustre 

 appears in the eye. 



Oblique illumination is used with advantage for investigating the anterior chamber. A 

 bright beam of light, condensed by a convex lens, is thrown laterally upon the cornea into the 

 eye, and so directed upon the point to be investigated as to illuminate it. A point so illumi- 

 nated, e.g., a part of the iris, may be examined from a distance by means of a lens, or even by 

 a microscope (Liebreich). 



The Orthoscope. Czermak constructed this instrument, in which the eye is placed under- 

 water (fig. 555). It consists of a small glass trough with one of its walls removed. The 

 margins of the open side are pressed firmly against the region of the eye. The eye and its sur- 

 roundings form, as it were, the sixth side of the trough, which is filled with water, so that the 



