306 NATURAL THEOLOGY. 



Let us endeavour to explain this, a, a beam 

 of light, being composed of the different coloured 

 rays, passes through the prism b. (See Fig. p. 

 296.) Instead of passing onward in a straight 

 line, it is refracted to c in distinct, and, conse- 

 quently, coloured rays. Whilst the whole of them 

 are bent or refracted at an angle from the dotted 

 line, they are also diverging from one another. 

 Their deviation from the straight line is their re- 

 fraction : their diverging from each other is their 

 dispersion. These properties being distinct, it is 

 conceivable that glass of a different chemical com- 

 position may affect the one to a greater degree 

 than the other, and, therefore, that a lens may be 

 composed of different kinds of glass, (crown-glass 

 and flint-glass, for example,) so that the con- 

 vergence of the rays into a focus may be obtained 

 without the dispersion of the rays, and the conse- 

 quent production of false colours round the image. 

 This is what Dollond nearly accomplished, and 

 upon these principles. That the effect of this 

 very artificial arrangement is attained in the eye, 

 is a remarkable proof of the perfection of its 

 adaptation to the properties of light. 



The last circumstance which we may mention 

 in continuing the comparison, is the drawing out 

 of the tube in the telescope to accommodate the 

 foci of the glasses to the distance of the object. 

 It is sufficient to say that the eye possesses this 

 property of accommodation. That we do not un- 



