346 PHYSIOLOGICAL PHYSICS. [Chap. xxvn. 



refract rays of light without, at the same time, dis- 

 persing them. In 1733 Hall, of Worcestershire, was 

 able to construct a lens which refracted rays of light 

 without dispersing them. He did not make known 

 his discovery. In 1757 a London optician, named 

 Dollond, rediscovered the method of getting rid of the 

 chromatic aberration. Lenses constructed with this 

 object are called ACHROMATIC. 



Suppose two prisms of the same material and pre- 

 cisely the same in other ways, and suppose a ray of 

 light to fall on one, it will be bent out of its course 

 and split up into a spectrum, in other words, dispersed. 

 The second prism will refract and disperse the ray to 

 the same extent. If, then, the second prism be placed 

 so as to receive the rays emerging from the first, but 

 placed with its refracting edge in the opposite direc- 

 tion, it will refract and disperse the rays to the same 

 extent as the first, but in an opposite direction. It 

 will, that is to say, exactly neutralise the action of the 

 first one, and the rays will be recombined and will 

 emerge from it in a direction parallel to that in which 

 they entered the first one. It was found, however, 

 that the extent to which a prism refracted a ray of 

 light was not necessarily a measure of the extent to 

 which it dispersed the ray. In other words, one might 

 have two prisms of different materials, so constructed 

 that while both dispersed a ray to the same extent, 

 one refracted less than the other. So that if these 

 two prisms were placed in opposite directions, the 

 second one would disperse equally with the first, but 

 in an opposite direction, and so reunite the dispersed 

 rays ; but it would refract less than the first, so that 

 the ray would emerge from the second prism unclis- 

 persed but not parallel to the entering ray, since all 

 the refraction produced by the first prism was not re- 

 versed by the second. It would still be refracted to an 

 extent equal to the difference between the refractions 



