146 THE MICROSCOPE. 



focus of objectives. As an improvement, Dr. Hooke suggested the 

 substitution of two fine hairs stretched parallel one to the other. 



1666. Auzont and Picard described a micrometer made of sil- 

 ver wire. 



When Hooke was shown Gascoigne's instrument, he gave a 

 sketch and description of it with several improvements (Phil. 

 Trans. Vol. i, p. 217). (Other methods of Hooke, see Posthumous 

 works, p. 497.) 



1662. Marquis of Malvasia, used the method for measuring 

 afterwards published by Auzont and Picard, saving that the latter 

 sometimes substituted silken thread, very fine, for the silver wire. 



Dechales and De la Hire both used same micrometers as 

 described. Later, De la Hire recommended the drawing of circles 

 on glass for telescopic uses. 



Leuwenhoek's method of measurement, crude though it was, 

 came quite near accuracy. He estimated size of objects by com- 

 paring them with grains of sand .scattered on plate with object. He 

 considered 100 grains in line to constitute an inch. By this 

 method he decided the red blood corpuscle to measure y^Vu i"ch. Dr. 

 Jurim's method was quite similar to his distinguished contemporary, 

 Leuwenhoek. He found the diameter of a fine silver wire by wrap- 

 ping it closely about a small cylinder for the length of an inch. 

 Then counting his wraps he obtains its thickness. He then cuts 

 this up into small pieces and uses it the same as Leuwenhoek does 

 the sand. 



Martin recommends the ruling on glass of parallel lines at 

 known distance apart and placing this in focus of eye-piece. 



Hooke viewed the objects with one eye and a graduated 

 measure placed at same distance with other. Thus by casting the 

 image, as it were, on the ruler, he was able to approximate correct, 

 ness of dimensions. 



1625. Erasmus Bartholin first noticed the peculiar effect upon 

 light by Iceland spar. He made quite a number of observations ou 

 objects viewed through this medium. 



M. Huygens, died 1695, made many additional and valuable 

 observations, being much more accurate in his descriptions than his 

 predecessor. He found that double refraction, was not peculiar to 

 Iceland spar, but was possessed in less marked degree by several 

 other substances. 



