188 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



convex form, which was found by Layard in Sargon's palace at Nim- 

 roud, was actually intended for use as a lens, in spite of Sir David 

 Brewster's contrary opinion. Nor can it be safely affirmed from their 

 minuteness of detail and perfection of execution that the finely en- 

 graved gems of antiquity were cut under lenses. Pliny the elder 

 and others state that globes filled with water were used as burning 

 glasses, and Seneca remarks that " letters though small and in- 

 distinct are seen enlarged and more distinct through a globe of glass 

 filled with water." Yet while defects of vision were frequently dis- 

 cussed by many classic authors, they made no reference to the 

 simplest optical aids, and .myopia was repeatedly declared to be in- 

 curable down to the end of the thirteenth century, when spectacles 

 first came into use. 



Roger Bacon and his teacher, Grossteste, undoubtedly understood 

 some of the properties of lenses and concave mirrors, but the evidence 

 advanced to support the opinion that Bacon used telescopes for astro- 

 nomical observations is not convincing. The early history of the 

 telescope remains rather obscure, but from our point of view the 

 most important fact is its application in astronomy by Galileo and 

 the revolution in human thought effected by his discoveries. His 

 sudden recognition and utilization of a principle which had certainly 

 been applied in the case of spectacles for 300 years quickly trans- 

 formed the equipment of the observatory and laid the foundation of 

 astrophysical research. In 1630 Francesco Generini saw the feasibil- 

 ity of using the telescope for increased precision in pointing, pre- 

 sumably by introducing threads into the focal plane of the eyepiece. 

 About 10 years later the inventor of the micrometer undoubtedly 

 used this method. The modern period of astronomical measure- 

 ment was thus begun. 



As for the telescope itself, it was first improved by the invention 

 of the Keplerian eyepiece and then increased in focal length to over- 

 come the troublesome effects of aberration. Rayleigh has shown that 

 a single lens of 1.7-inch aperture is as good as an achromatic when 

 its focus is G6 feet. Huygens, who worked out the theory of aber- 

 ration, consequently greatly increased the aperture and focal length 

 of his telescopes. He also devised the Huygenian eyepiece and was 

 rewarded for his efforts by the discovery of the true nature of the 

 rings of Saturn. Three of his objectives, with focal lengths of 122, 

 170, and 210 feet, respectively, are still in the possession of the Royal 

 Society. Telescopes up to 600 feet in length were made in this 

 period, but the difficulty of finding and following the celestial object 

 seriously affected their value. Obviously, they could not be carried 

 on equatorial mountings, first described for telescopic purposes in 

 Schemer's Rosa Ursina, but really not different in principle from 

 the equatorial armilla of Tycho Brahe. A n accessory of the highest 



