OPTICAL IMAGES. 



CHAPTER III. 



63. Experimental illustration. 64. Dispersive powers. 65. Dispersive 

 power does not necessarily increase with refractive power. 66. 

 Example of the diamond. 67. Achromatic lens. 68. Achromatic 

 combination of flint and crown-glass. 69. Form of the compound 

 lens. 



63. To make this, which, is a circumstance of the highest 

 importance, more clear, let L L, fig. 32, and if I/, fig. 33, be two 

 lenses, the former of diamond, and the latter of glass, and let o o 

 and o' o' be a white object placed at the same distance before them. 

 Let v be the violet, and n the red image, produced by the lens 

 L L, the images of the intermediate colours being between v and B, 

 according to what has been explained above. Now let us suppose 

 that such a convexity is given to the lens L' L', which is evidently 

 always possible, that the distance of the violet image v' of o' o' 

 from the lens i/ L' shall be equal to that of the violet image v of o o 



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No. 84. 



