98 Britain's Heritage of Science 



degree in their passage through a lens, come to a focus at 

 different points. Hence the images formed by simple lenses 

 of glass are coloured. Though the possibility of combining 

 several lenses made of different materials had occurred to 

 Newton, he had come to the conclusion that the dispersive 

 power of substances (which is the power to separate different 

 colours), is proportional to their refractive power, and if 

 this were really the case, it would indeed be impossible to 

 construct a lens which could bring different coloured rays 

 to the same focus. The succeeding history of the subject is 

 interesting. Euler asserted that notwithstanding Newton's 

 experiments, which he accepted, it should be possible to 

 produce achromatism, i.e., images without coloration, by 

 means of a combination of lenses. David Gregory had 

 already in 1695 expressed similar ideas, and their argument 

 depended on the belief that the images formed by the human 

 eye are not deteriorated by any colour-dispersion. As the 

 rays entering the eye are concentrated on the retina by 

 successive refraction through different media, such as the 

 cornea, the crystalline lens and the vitreous humour, it 

 was argued that it should be possible to produce achromatic 

 images by properly combining lenses of different materials. 

 Euler's belief that the optical arrangement of the eye pointed 

 the way to the construction of achromatic lenses was shared 

 by others, and ultimately led to the solution of the problem ; 

 but the curious point is, that the premise on which the whole 

 argument depends is wrong, the eye not being achromatic 

 at all, but subject to the same defects as a simple lens. 



A Swedish mathematician, Klingenstjerna, seems to have 

 been the first to repeat Newton's experiments with sufficient 

 care, when it appeared that the relationship between 

 refractive and dispersive powers, which Newton thought 

 he had established, did not hold accurately. John Dollond 

 (1706-1761), a son of one of the many French refugees who 

 came to England after the revocation of the Edict of Nantes, 

 had started life as a silk weaver in Spitalfields, but relin- 

 quished this occupation and established a workshop for 

 optical instruments. Having heard of Klingenstjerna's obser- 

 vation, he entered into an independent investigation on the 

 opticaL properties of different kinds of glass, and had the 



