100 Things not generally Known. 



lescopes that should magnify 4000 times, by means of which 

 the lunar mountains might be accurately laid down. 



Optical instruments of such enormous focal lengths remind 

 us of the Arabian contrivances of measurement : quadrants with 

 a radius of about 190 feet, upon whose graduated limb the 

 image of the sun was received as in the gnomon, through a 

 small round aperture. Such a quadrant was erected at Sa- 

 marcand, probably constructed after the model of the older 

 sextants of Alchokandi, which were about sixty feet in height. 



LATE INVENTION OF OPTICAL INSTRUMENTS. 



A writer in the North-British Review, No. 50, considers it 

 strange that a variety of facts which must have presented 

 themselves to the most careless observer should not have led 

 to the earlier construction of Optical Instruments. The an- 

 cients, doubtless, must have formed metallic articles with con- 

 cave surfaces, in which the observer could not fail to see him- 

 self magnified ; and if the radius of the concavity exceeded 

 twelve inches, twice the focal distance of his eye, he had in 

 his hands an extempore reflecting telescope of the Newtonian 

 form, in which the concave metal was the speculum, and his 

 eye the eye-glass, and which would magnify and bring near him 

 the image of objects nearly behind him. Through the spheri- 

 cal drops of water suspended before his eye, an attentive ob- 

 server might have seen magnified, some minute body placed 

 accidentally in its anterior focus ; and in the eyes of fishes and 

 quadrupeds which he used for his food, he might have seen, 

 and might have extracted, the beautiful lenses which they 

 contain, and which he could not fail to regard as the principal 

 agents in the vision of the animals to which they belonged. 

 Curiosity might have prompted him to look through these re- 

 markable lenses or spheres ; and had he placed the lens of the 

 smallest minnow, or that of the bird, the sheep, or the ox, in 

 or before a circular aperture, he would have produced a micro- 

 scope or microscopes of excellent quality and different magni- 

 fying powers. No such observations seem, however, to have 

 been made ; and even after the invention of glass, and its con- 

 version into globular vessels, through which, when filled with 

 any fluid, objects are magnified, the microscope remained un- 

 discovered. 



A TRIAD OF CONTEMPORARY ASTRONOMERS. 



It is a remarkable fact in the history of astronomy (says 

 Sir David Brewster), that three of its most distinguished pro- 

 fessors were contemporaries. Galileo was the contemporary 

 of Tycho during thirty-seven years, and of Kepler during the 

 fifty-nine years of his life. Galileo was born seven years before 



