SECT. XIX. ACHROMATIC TELESCOPE. 165 



the other end to view the image thus formed. Now it is found 

 that the object-glass, instead of making the rays converge to 

 one point, disperses them, and gives a confused and coloured 

 image : but by constructing it of two lenses in contact, one of 

 flint and the other of crown-glass of certain forms and propor- 

 tions, the dispersion is counteracted, and a perfectly well-defined 

 and colourless image of the object is formed (N. 196). It was 

 thought to be impossible to produce refraction without colour, 

 till Mr. Hall, a gentleman of Worcestershire, constructed a tele- 

 scope on this principle in the year 1733 ; and twenty-five years 

 afterwards the achromatic telescope was brought to perfection by 

 Mr. Dollond, a celebrated optician in London. 



By means of Mr. Fraunhofer's determination of the refraction 

 of the principal rays in substances, their dispersive powers may 

 be found (N. 197). 



A perfectly homogeneous colour is very rarely to be found ; 

 but the tints of all substances are most brilliant when viewed in 

 light of their own colour. The red of a wafer is much more 

 vivid in red than in white light ; whereas, if placed in homoge- 

 neous yellow light, it can no longer appear red, because there is 

 not a ray of red in the yellow light. Were it not that the wafer, 

 like all other bodies, whether coloured or not, reflects white light 

 at its outer surface, it would appear absolutely black when 

 placed in yellow light. 



After looking steadily for a short time at a coloured object, 

 such as a red wafer, on turning the eyes to a white substance, a 

 green image of the wafer appears, which is called the accidental 

 colour of red. All tints have their accidental colours : thus the 

 accidental colour of orange is blue ; that of yellow is indigo ; 

 of green, reddish white ; of blue, orange-red ; of violet, yellow ; 

 and of white, black ; and vice versa. When the direct and 

 accidental colours are of the same intensity, the accidental 

 is then called the complementary colour, because any two 

 colours are said to be complementary to one another which pro- 

 duce white when combined. 



From experiments by M. Plateau of Brussels, it appears that 

 two complementary colours from direct impression, which would 

 produce white when combined, produce black, or extinguish one 

 another, by their union, when accidental ; and also that the com- 

 bination of all the tints of the solar spectrum produces white 



