A HISTORY OF SCIENCE 



constructed an instrument not very unlike that of 

 Hadley; but this invention was lost sight of until 

 several years after the philosopher's death and some 

 time after Hadley 's invention. 



The introduction of the sextant greatly simplified 

 taking reckonings at sea as well as facilitating taking 

 the correct longitude of distant places. Before that 

 time the mariner was obliged to depend upon his com- 

 pass, a cross-staff, or an astrolabe, a table of the sun's 

 declination and a correction for the altitude of the pole- 

 star, and very inadequate and incorrect charts. Such 

 were the instruments used by Columbus and Vasco da 

 Gama and their immediate successors. 



During the Newtonian period the microscopes gen- 

 erally in use were those constructed of simple lenses, 

 for although compound microscopes were known, the 

 difficulties of correcting aberration had not been sur- 

 mounted, and a much clearer field was given by the 

 simple instrument. The results obtained by the use of 

 such instruments, however, were very satisfactory in 

 many ways. By referring to certain plates in this vol- 

 ume, which reproduce illustrations from Robert Hooke's 

 work on the microscope, it will be seen that quite a 

 high degree of effectiveness had been attained. And it 

 should be recalled that Antony von Leeuwenhoek, whose 

 death took place shortly before Newton's, had dis- 

 covered such micro - organisms as bacteria, had seen 

 the blood corpuscles in circulation, and examined and 

 described other microscopic structures of the body. 



