184 ACHROMATIC TELESCOPE. [SECT. xrx. 



two prisms may be made such that, when the prisms are 

 placed close together with their edges turned opposite ways, 

 they will exactly oppose each other's action, and will refract 

 the coloured rays equally, but in contrary directions, so that 

 an exact compensation will be effected, and the light will 

 be refracted without colour (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 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 proportions, the dispersion 

 is counteracted, and a perfectly well-defined and colourless 

 image of the object is formed (N. 192). It was thought to 

 be impossible to produce refraction without colour, till Mr. 

 Hall, a gentleman of Worcestershire, constructed a telescope 

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

 afterwards, the achromatic telescope was brought to per- 

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



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 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 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; 



