THE RETINA. 1 09 



Astigmatism. This term is applied to a condition of the eyes in which the 

 curvature of the cornea is somewhat less in the vertical than in the horizontal 

 meridian. If such an eye is so accommodated that rays which lie in the 

 vertical meridian converge to the retina, while those in the horizontal meridian 

 converge beyond it, the point is seen as a horizontal bar. 



Chromatism. If the eye is fixed on a luminous point at a great distance, it 

 often appears as if it had a sharply defined red centre, surrounded by a luminous 

 fringe. This happens when the accommodation is such that the less refrangible 

 rays converge to the retina, the more refrangible in front of it. In the contrary 

 case, i.e. when the distance of the luminous point is less than that for which 

 the eye is accommodated, the centre is blue. 



Entommatic vision. Shadows of objects floating in the media of the eye 

 are distinguished by the retina, when the eye is illuminated from a point which 

 is so near the cornea that the rays in entering the eye become parallel. As 

 thus seen, objects behind the pupil may be distinguished from objects in front 

 of it, by the observation that they appear to move in the direction opposite to 

 that in which the source of light is moved. 



Reflection of light in the Eyeball. Light which reaches 

 the retina is partly absorbed, partly reflected. Every re- 

 flected ray returns approximately in the path of its incidence. 

 Consequently, although the cavity of the globe appears 

 under ordinary circumstances dark, it can be made to 

 appear luminous if illuminated by light which reaches it as 

 if it came from the eye of the observer. Thus, if a plate 

 of glass be so placed between the observing and the 

 observed eye that light emanating from a luminous source 

 is reflected by it into the observed eye, as if it came from 

 the other, the former appears bright ; and if it were possible 

 for both eyes to remain relaxed, that is accommodated for 

 extreme distance, a distinct image of the retina would be 

 seen by the observing eye. In the ophthalmoscope as 

 originally invented by Helmholtz, this is done by the 

 interposition of a concave correcting lens between the 

 observing eye and the mirror. 



The Retina. The retina is a sensory end-organ excited 

 by light. Its excitability has its exclusive seat in the 

 bacillary layer. 



If a strong light is suddenly thrown, by a lens, on the outside of the eyeball 

 of a person in a dark room, an appearance of branching blood-vessels is seen 

 by him, of which the explanation is that the side light throws shadows of the 



