CHAPTER X. 



VISION AND OPTICAL ILLUSIONS THE EYE DESCRIBED ACCOMMODATION 

 OF THE EYE CHROMATIC ABERRATION SPINNING TOPS. 



THE eye is an optical instrument that may be compared with those con- 

 structed by physicists themselves ; the media of which it is composed 

 have surfaces like those which enter into the construction of optical instru- 

 ments. It was Kepler who at the end of the eighteenth century discovered 

 the passage of light into the eye. Soon after the discovery of the inner 

 chamber he found that the eye realized the conditions that Porta had com- 

 bined to obtain the reflection of external objects. 



We will now briefly state that the coats of this organ are constituted of 

 a fibrous membrane, T (fig. 95), termed sclerotic, which is opaque, except in the 



anterior portion of the eye, where it forms 

 the transparent cornea. The crystalline, C, 

 enshrined behind the cornea, is the con- 

 vergent lens of the inner chamber ; it is 

 covered with a transparent membrane, 

 /lp or capsule, and is bathed in two fluids, the 

 aqueotis humour, between the crystalline 

 humour and the cornea, and the vitreous 

 body, a gelatinous humour lodged between 

 the crystalline and the back of the eye. 



Fig. 9 5.-Structureoftheeye. The image Q f exterior objects which is 



produced by the passage of light through these refracting surfaces, is 

 received by a nervous membrane, the retina, B, formed by an expansion of 

 the optic nerve, N. We must also mention the choroid, a membrane lined 

 with a dark pigment, which absorbs the light, and prevents interior reflec- 

 tions, and in front of the crystalline lens, a curtain with an opening, H, 

 called the iris, which gives to the eyes their colour of blue, grey, or black. 

 The opening in the centre of the iris is called the pupil. 



The penetration of light through the surfaces of the eye is easily 

 demonstrated. An object throws divergent rays on the cornea, a part pene- 

 trates into the eye and falls upon the retina, leaving a perfectly retained, 

 image of the object. Magendie has proved in the following manner the truth 

 of this mathematical deduction. The eye of a rabbit is very similar to an 



