OPTICS. 



three original colours, red, blue, and 

 yellow: tor the orange being situated be- 

 tween the red and yellow, is only the 

 mixture of these two : the green, in like 

 manner, arises from the blue and yellow ; 

 and the violet from the blue and red. 



As the colour of a body, therefore, pro- 

 ceeds from a certain combination of the 

 primary rays which it reflects ; the com- 

 bination of rays flowing from any point of 

 an object will, when collected by a 

 glass, exhibit the same compound colour 

 in the corresponding point -of the image. 

 Hence appears the reason why the 

 images formed by glasses have the co- 

 lours of the objects which they represent. 



The instance of the separation of the 

 primary colours of light which seems 

 most remarkable, is that of the RAIN- 

 BOW. It is formed, in general, by the re- 

 flection of the rays of the sun's light 

 from, the drops of falling rain, though 

 frequently it appears among the wuves of 

 the sea, whose heads, or tops, are blown 

 by the wind into small drops, and it is 

 sometimes seen on the ground, when the 

 sun shines on a very thick dew. Cas- 

 cades and fountains, whose waters are in 

 their fall divided into drops, exhibit 

 rainbows to a ^pectator, if properly si- 

 tuated during the time of the sun's 

 shining ; and water blown violently from 

 the mouth of an observer, whose back is 

 turned towards the sun, will, with care, 

 produce the same phenomenon. See 

 RAINBOW. 



This appearance is also seen by moon- 

 light, though seldom vivid enough to 

 render the colours distinguishable ; and 

 the artificial rainbow may be produced 

 even by candle-light, on the water which 

 is ejected by a small fountain, or jet d'eau. 

 All these are of the same nature, and de- 

 pendent on the same causes, viz. the va- 

 rious refrangibility of the rays of light. 



The colours observable on soap-bub- 

 bles, and the halos which sometimes sur- 

 round the moon, are also referable to the 

 same origin. 



" Of the Eye, and the nature of Vision." 

 The eye is nearly of a spherical shape, 

 and is composed of three different sub- 

 stances, called, 1. The aqueous, P. (fig. 

 17). 2. The crystaline, R ; and 3. The 

 vitreous humours, V, enclosed by three 

 principal coats, which are formed by the 

 expansion of the different component parts 

 of the optic nerve, viz. the sclerotica, S. S. 

 2. The choroides, D D ; and 3. The retina, 

 T T. The sclerotica is outermost ; 

 it is very strong, and the forepart, which 

 is transparent, and somewhat prominent, 



is called the cornea, C. The choroides is 

 next in order, and lias a circular perfora- 

 tion, P, called the pupil, immediately be- 

 hind the middle of the cornea : the part 

 11. of the choroides, visible behind the 

 cornea, is flat ; it is called the iris, or uvea, 

 and is differently coloured in different 

 persons. The retina is the inmost coat, 

 it extends round the eye till it meets the 

 ciliary ligaments, QQ, membranes pro- 

 ceeding from the choroides, and attached 

 to the capsula or filament, which encloses 

 the crystaline humours, R. The crysta- 

 line is the most dense of the three hu- 

 mours, and is in the shape of a double 

 convex lens, whose forepart has the less 

 curvature ; the cavity between the cornea 

 and the crystaline is occupied by the 

 aqueous humour, which has rather the 

 least density of the three, and the 

 space between the bottom of the eye and 

 the crystaline is filled by the vitreous 

 humour, V. 



Objects presented to the eye have their 

 images painted on the back part of the 

 retina, the rays of the incident pencils 

 converging to their proper foci there by 

 the refraction of the different humours ; 

 and for this office they are admirably 

 adapted ; for as the distance between 

 the back and front of the eye is very 

 small, and the rays of each of the pencils 

 that form the image fall parallel, or else 

 diverging on the eye, a strong refractive 

 power is necessary for bringing them to 

 their foci at the retina ; but each of the 

 humours, by its peculiar form and densi- 

 ty, contributes to cause a convergence of 

 the rays ; the aqueous from its convex 

 form ; the crystaline by its double con- 

 vexity and greater density than the 

 aqueous; and the vitreous by a less 

 density than the crystaline joined to its 

 concave form. 



These things are manifest from what 

 has been already said. The structure of 

 the eye is in general adapted to the re- 

 ception of parallel rays ; but as the dis- 

 tances of visible objects are various, so 

 the eye has powers of accommodating 

 itself to rays proceeding from different 

 distances, by altering the distance of the 

 crystaline from the retina, which is done 

 by the action of the ciliary ligaments. 



That this change of situation in the 

 crystaline is adequate to such accommo- 

 dation, may be thus shewn. Suppose a 

 pencil of rays to diverge from a point, A, 

 (fig. 18.) at a distance from the eye less 

 than that which admits distinct vision in 

 the usual situation of the humours : the 

 rays would come to a focus, V, behind 



