18 THE ACHROMATIC LENS. 



object, and the eye placed six inches behind this 

 magnified image will see it with the greatest dis- 

 tinctness. Now in this case the image is magnified 

 four times directly by the lens, and 200 times by 

 being brought 200 times nearer the eye ; so that 

 its apparent magnitude will be 800 times larger 

 than before.* 



Not thirty years ago, it was asserted by men of 

 no less eminence than Dr. Wollaston and M. Biot, 

 that the compound microscope would never rival 

 the single one ; and for this conclusion there appeared 

 to be weighty reasons. And yet existing difficulties, 

 which pressed heavily on their minds, have all been 

 surmounted. More than one name is entitled to 

 honour in this service; especially the late Mr. Tulley, 

 of London, who was induced by Dr. Goring to 

 attempt the construction of an achromatic^ object 

 glass for a compound microscope. 



The white light which comes from the sun, or 

 any other luminous body, is composed of seven dif- 

 ferent kinds of light red, orange, yellow, green, 

 blue, indigo, and violet. Mr. Tulley, therefore, 

 produced a glass composed of three lenses, which 

 gave to the vision a distinctness not previously at- 

 tained, by combining the colours of which light is 

 composed, and causing objects to be seen in white 

 light. Great importance was once attached to 



* Brewster on Optics, pp. 45 4?8. 



f An optical term derived from the Greek, and signifying " without 

 colour." 



