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diately at the brain. A black skin lines the whole in- 

 side of the globe : at the fore part of it is ? round 

 orifice, which contracts or dilates itself according as the 

 light is more or less strong. Six muscles, which are 

 placed on the outside of the globe, move different ways, 

 and the rapidity of those motions is excessive. 



What need is there of these humours, this cloth, 

 this tapestry, this aperture which contracts and dilates 

 itself? The light comes to us from the SUD, in a light 

 line ; but these rays become crooked, when the density 

 of the mediums through which they pass increases or di- 

 minishes : this is called the refraction of light. 



To the property of refracting light, joins that of re- 

 flecting from the body it enlightens. There issue then 

 luminous streaks from all points of the objects, which 

 bear the image of these points. 



The humours of the eye are the lens of the camera 

 obscura : the cloth, or retina, are the pasteboard : the 

 black skin which hangs within the ball performs the 

 office of a shutter that excludes the light; it extin- 

 guishes the rays whose reflection would render the 

 image less distinct ; the loll, by contracting or dilating 

 itself in proportion to the strength of the light, mo- 

 derates the action of the rays on the retina : the nerve, 

 placed behind this, communicates to the brain the 

 various concussions it receives, to which divers percep- 

 tions correspond. 



5. Such are the admirable relations which WISDOM 

 has placed between our eyes and the light; those which it 

 has established between light and the surfaces of differ- 

 ent bodies, whence colours proceed, are not less worthy 

 our attention. 



A- ray which falls en a glass prism, divides into seve- 

 ral principal rays, each of which bear its proper colour. 

 The oblong image which this refraction produces, 

 affords several coloured stripes, distributed in a regular 

 order : the first, reckoning from the upper part of the 

 image, is red, the second orange, the third yellow, the 

 fourth green f the fifth blue, the sixth indigo, the 



