764 



ILLUMINATION OF THE EYE. 



with an ascending current, that with opening [breaking] the current with a descending current. 

 If a uniform continuous ascending current be transmitted through the closed eyes, the dark disc 

 of the elevation at the entrance of the optic nerve appears in a whitish-vto^ field of vision ; 

 with a descending current, the field of vision is reddish and dark, in which the position of the 

 optic nerve appears light blue (v. Helmholtz). If external colours are looked at simultaneously, 

 these colours blend to form a violet or yellow with the colours looked at (Schelske). During 

 the passage of the ascending current we see external objects indistinctly and smaller when the 

 eyet art* open ; while with the descending current they are larger and more distinct (Ritter). 

 Sometimes the position of the macula lutea appears dark on a bright ground, or the reverse, 

 according to the direction of the current. If the current be opened [broken] the phenomena 

 are reversed ( 335), and the eye soon returns to rest. 



11. The yellow spot apjwars sometimes as a dark circle when there is a uniform blue illumi- 

 nation. In a strong light the position of the yellow spot is surrounded by a bright area, twice 

 or thrice as large, called "Lowe's ring." [Clerk-Maxwell's Experiment. On looking through 

 a solution of chrome-alum in a bottle or vessel with parallel glass sides, we observe an oval 

 purplish spot in the greenish colour of the alum. This is due to the pigment of the yellow spot] 



Haidinger's Brushes. On directing the eye towards a source of polarised light, " Haidinger's 

 jwlarised brushes " appear at the point of fixation. They are seen on looking through a Nicol's 

 prism at a bright cloud {v. Helmholtz). They are bright and bluish on a surface, bounded by 

 two neighbouring hyperbola on a white field ; the dark bundle separating them is smallest in 

 the centre and yellow. Of the various colours of homogeneous light, blue alone shows the 

 brushes (Stokes). According to v. Helmholtz the seat of the phenomenon is the yellow spot, 

 and is due to the yellow-coloured elements of the yellow spot being slightly doubly refractive, 

 while at one part they absorb more, at another less, of the rays entering the eye. 



12. Lastly, there arc the visual sensations depending on internal causes, e.g., increased 

 bounding of the blood through the retina, as during violent coughing, increased intraocular 

 pressure. Stimulation of the visual areas ( 378, IV.) may produce spectra, which Cardanus 

 (1550), Goethe, Nicolai, and Johannes Muller could produce voluntarily. 



394. ILLUMINATION OF THE EYE. OPHTHALMOSCOPE. The light 

 which enters the eye is partly absorbed by the black uveal pigment, and partly 

 again reflected from the eye, and always in the same direction in which the rays 

 entered the eye. By placing oneself in front of the eye of another person, of course 

 the head, being an opaque body, cuts off a large number of rays. Owing to the 



position of the head, no rays of light can enter 

 the eye ; and of course none can be reflected 

 back to the eye of the observer. Hence, the 

 eye of the person being examined always appears 

 black, because those rays which alone could be 

 reflected in the direction of the eye of the ob- 



I 



Fig. 548. 



Arrangement for examining the eye of B. A, eye of observer ; x, source of light ; S, S, plate 



of glass directed obliquely, reflecting light into B. 



server are cut off. As soon, however, as we succeed in causing rays of light to 

 enter the eye at the same time and in the same direction in which we observe the 

 eye of another person, the fundus of the eye appears brightly illuminated. 

 The following simple arrangement is sufficient for the purposes (fig. 548) : Let B be the eye 



