PHYSICS OF THE NINETEENTH CENT. OPTICS. 515 



are immense. As we have already seen, the simple microscope 

 did wonders in the hands of Leeuwenhoek, and enabled Malpighi, 

 Grew, Swammerdam, and others to discover much ; and although some 

 forms of compound microscopes had been constructed even as early 

 as the beginning of the seventeenth century, they were mere curiosities, 

 for it was not until about the first quarter of the present century that 

 methods were discovered of correcting the spherical and chromatic 

 aberrations of the lenses. In 1821 we find Biot stating " that opticians 

 regard the construction of a good achromatic microscope as impos- 

 sible," and Wollaston declared that "the compound instrument would 

 never rival the single." However, the subject was pursued by some 

 excellent practical opticians, among whom we may name Fraunhofer 

 at Munich, Amici at Modena, Chevalier at Paris, and M. Selligues. 

 A microscope with an achromatic object-glass of several lenses, made 

 by Chevalier, was presented to the Academy of 

 Sciences in 1824. In the same year Tulley of 

 London succeeded in constructing for the first 

 time in England an object-glass of three lenses. 

 Sir John Herschel, Professor Airy, and Pro- 

 fessor Barlow furnished valuable contributions 

 to the theory of the achromatic object-glass. 

 More recently a suggestion of Sir David Brew- 

 ster's has been carried out by the construction 

 of lenses of diamond. By these and other 

 modern improvements, especially in the mode 

 of illuminating the objects, investigations are 

 now carried into structures so minute that mag- 

 nifying powers of 2,000 or 3,000 diameters 

 have to be used. The applications of polarizing 

 and spectroscopic apparatus to the microscope 

 were further additions to its powers. The bi- 

 nocular form of construction, though attempted 

 very long ago, was not successfully carried out 

 until 1851. A microscope of this construction, 

 one model of which is represented in Fig. 234, 

 has the advantage of causing less fatigue and 

 strain to the eye, and of showing the relief of 

 objects under low powers. 



The great advances in the construction of astronomical telescopes 

 have been particularly alluded to in the chapter recording the recent 

 progress of astronomy. A brief summary of what is there stated may 

 be inserted here for the sake of continuity. The improvements in the 

 manufacture of optical glass have enabled large and perfect discs to 

 be produced, and achromatic object-glasses of a very large size 2 feet 

 and more in diameter have been constructed. In reflecting tele- 

 scopes the heavy metallic mirror has always presented many incon- 



332 



FIG. 234. BINOCULAR 

 MICROSCOPE. 



