280 HISTORY OF SCIENCE. 



variously coloured rays of the spectrum. The diagram, Fig. 132, shows 

 the effect of a convex lens in separating rays of white light into its con- 

 stituent coloured rays. According to the order of their refrangibility 

 the rays are bent more or less out of their course, and it will be ob- 

 served that as they are more refrangible they cross the axis of the lens 

 at points nearer to it ; hence the images formed are always more or 

 less indistinct in their outlines, and exhibit fringes of colour. These 

 colours are very noticeable when the image formed by a lens is viewed 

 by another lens, as in the telescope ; and this circumstance much im- 

 paired the efficiency of the earlier astronomical telescopes. Newton 



RED 



YELLOY/ 



BLUE 



FIG. 132. 



made some experiments from which he was led to trie conclusion that 

 it would be impossible by any combination of materials in a lens to 

 obtain the requisite refraction without the separation of the coloured 

 rays; but in 1729 Mr. Charles More Hall was convinced that the 

 optical arrangement of the human eye would afford some clue to the 

 solution of the problem, and it is said that he succeeded in producing 

 lenses that formed images free from colour. It was affirmed towards 

 the end of last century that several telescopes made under Mr. Hall's 

 directions were in existence. It is certain that Mr. Hall's discovery 

 attracted so little notice that it was generally unknown, and the person 

 with whose name the achromatic lens is always associated, discovered 

 it for himself in the manner to be presently related. It may be first 

 stated that the celebrated Euler also had, in 1747, arrived at the con- 

 clusion that it was possible to make an achromatic lens by imitating 

 the construction of the eye. He proposed to form the lenses of glass 

 enclosing liquids, so that the refraction might be produced as in the eye. 

 All attempts to carry Euler's system into practice proved to be com- 

 plete failures ; and, in fact, P^uler himself afterwards admitted that his 

 mathematical analysis overlooked some of the real physical conditions 

 of the problem. It happened curiously enough that the inventor of 

 the now well-known achromatic combination was led to it by a con- 

 troversy with Euler, in which he endeavoured to combat the conclusions 

 of that eminent mathematician as to the possibility of constructing 

 achromatic lenses. 



JOHN DOLLOND was born in London in the year 1706. His father 



