SJCT. XIX. THE ACHROMATIC TELESCOPE. 159 



flint glass is much greater than the space occupied by 

 that produced by the crown glass ; and as the quantity 

 of dispersion depends upon the refracting angle of the 

 prism, the angles of the two prisms may be made such, 

 that when the prisms are placed close together with tbjeir 

 edges turned opposite ways, they will exactly oppose 

 each other's action, and will refract the colored rays 

 equally but in contrary directions, so that an exact com- 

 pensation will be effected, and the light will be refracted 

 without color (N. 191). The achromatic telescope is 

 constructed on this principle. It consists of a tube with 

 an object glass or lens at one end to bring the rays to a 

 focus and form an image of the distant object, and a 

 magnifying glass at the other end to view the knage 

 thus formed. Now it is found that the object-glass, 

 instead of making the rays converge to one point, dis- 

 perses them, and gives a confused and colored image : 

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

 flint and the other of crown glass of certain forms and 

 proportions, the dispersion is counteracted, and a per- 

 fectly well defined and colorless image of the object is 

 formed (N. 192). It was thought to be impossible to 

 produce refraction without color, till Mr. Hall, a gentle- 

 man of "Worcestershire, constructed a telescope on this 

 principle in the year 1733 ; and twenty-five years after- 

 ward, the achromatic telescope was brought to perfec- 

 tion by Mr. Dollond, a celebrated optician in London. 



A perfectly homogeneous color is very rarely to be 

 found, but the tints of all substances are most brilliant 

 when viewed in light of their own color. The red of a 

 wafer is much more vivid in red than in white light ; 

 whereas if placed in homogeneous 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 colored 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 colored 

 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 color of red. All tints 

 have their accidental colors : thus the accidental color 



