98 THE PATH OF SCIENCE 



means of lenses and invented reflecting telescopes, using mir- 

 rors to avoid the difficulty with chromatic aberration. 



It was shown experimentally about the middle of the 

 eighteenth century that Newton had been wrong and that 

 achromatic lenses could be made. The whole subject was put 

 on a solid foundation by Fraunhofer, who in 1817 discovered 

 that in the solar spectrum there were certain dark lines that 

 enabled him to identify the positions of the colors of the 

 spectrum with accuracy and to measure with precision the 

 refractive indices of glass for light of different colors. 



Joseph von Fraunhofer was able to calculate the principles 

 required for the achromatism of the telescope and made an 

 excellent refractor of 9%-inch aperture to be used by the 

 astronomers of Dorpat Observatory. Fraunhofer also made 

 optical glass and was really the first working optical instru- 

 ment maker of the modern school. 



While the use of light in optical instruments w^as advanc- 

 ing, the nature of light continued to engage the minds of 

 men. Newton had devoted much thought to the dynamics 

 of particles, and it is not surprising that he considered light 

 to consist of material particles emitted from heated bodies 

 and producing a mechanical effect by their action on the eye. 

 A phenomenon observed by Francesco Grimaldi, however, 

 was difficult to reconcile with any theory that considered light 

 to consist of particles, that is, that if a point source of light 

 illuminates a sharp straight edge, such as a knife blade, the 

 shadow will be bounded by a series of light and dark bands. 

 To this phenomenon Grimaldi gave the very appropriate 

 name of diffraction^ by which it is still known. Diffraction 

 had also been observed by Robert Hooke, the energetic and 

 versatile secretary of the Royal Society, who concluded that 

 there was some kind of vibrating motion in light. Thus 

 Newton was induced to suggest that the corpuscles of light 

 embodied a vibratory element. The rays of light, for in- 

 stance, in passing by the edges of bodies might be bent back- 

 wards and forwards several times "with a motion like that of 

 an eel." 



