284 



HISTORY OF SCIENCE. 



faces touch the surfaces of the lens at the points where the ray enters 

 and leaves the lens. The ray thus refracted is also dispersed in the 

 direction F VP, F R T, and would of itself form a spectrum on a screen 

 at P T but for the flint-glass lens //; and the violet ray would cross the 

 axis of the lens at v, while the red would cross at a more distant point 



FIG. 134. 



R. But the flint-glass lens, or say the prism a AC, being interposed, 

 another direction is given to these rays, so that they meet at /; simi- 

 larly of the ray s F. These coincidences of the rays at one point f, 

 instead of at two points R and v, of course give a greatly improved 

 clearness and precision to the image of an object. 



Many trials had, of course, to be made before Dollond successfully 

 carried his idea into practice ; but such was his perseverance and in- 

 genuity, that it was not long before he produced refracting telescopes 

 giving images retaining scarcely any trace of the coloured borders which 

 formed so great a defect in the former telescopes. When the success- 

 ful issue of his efforts was announced, many eminent mathematicians 

 were unable to credit the report. The achromatic lens has been of 

 immense service in the advancement of science. The modern astro- 

 nomical telescope and the compound microscope owe their power to 

 this invention, and the practical advantages ob- 

 tained by the use of the achromatic lens in sex- 

 tants and surveying instruments are incalculable. 

 Dollond constructed his first achromatic tele- 

 scopes in 1758, and although these were more 

 efficient than the old telescopes of three times 

 their length, he extended his principle to the 

 construction of object-glasses composed of three 

 lenses. Such was the optical excellence of these 

 object-glasses, that one of 43 inches focal length 

 would have a magnifying power as great as could 

 be applied in a telescope of the old construction 

 50 feet in length. The annexed cut, Fig. 135, 

 exhibits sections of various achromatic lenses, 

 including triple lenses. For the discovery of the achromatic lens the 



FIG. 135. 



