THE PRESIDENT'S ADDRESS. 119 



this invention in much the same fashion as he did to that of the 

 telescope, for he never published any account of his invention till 

 1646, notwithstanding his assertion that he made the discovery a 

 quarter of a century before. We may fairly disregard his claim in 

 the undoubted fact that one of Jansen's microscopes which had 

 been presented to Prince Maurice was in the possession of Cornelius 

 Drebell, who, in 1617, resided in London, as mathematician to 

 King James VI. Eustace Divini, about this period, made 

 microscopes with two object glasses, as they were then called, and 

 two plano-convex eye glasses joined together on their convex sides, 

 enclosing them in a tube as large as a man's leg, the eye-pieces 

 being of the size of the palm of the hand ; but opticians were all 

 at sea in their conceptions of microscopical requirements, and hence 

 this clumsy and unwieldly tube of Divini's, for we find that about 

 1688 Hartsoeker, by means of a single lens of high curvature, made 

 such investigations that he laid the foundation for our true know- 

 ledge of the function of reproduction ; and those of you who have 

 any acquaintance with physiology will readily understand that the 

 minute single lenses he employed mnst have possessed great mag- 

 nifying power and a not very imperfect definition. The combination 

 of these qualities rendered them so suitable for the amplification of 

 small objects that books treating of microscopes, published about 

 this time, contain directions for the production of these lenses by 

 melting threads of glass in the flame of a candle till the glass runs 

 into a spherical drop. Christian Huygens, an eminent Dutch 

 mathematician and astronomer, about 1678, had made such a simple 

 microscope as this, the jqUi of an inch in diameter, which gave a 

 linear magnifying power of 100, and doubtless this was considered 

 an achievement in those days ; and except for the difficulty of 

 applying objects to it, the want of light, and the contracted field, 

 it might be considered a very perfect instrument for that time. It 

 was with such a microscope that Leeuwenhoek made all those mar- 

 vellous discoveries of infusorian life which will immortalise his 

 name wherever the microscope is used. He employed double 

 convex lenses of various diameters, which he made for himself by 

 melting rods of glass in a flame and afterwards grinding them to 

 the desired curvature. Twenty-six of these microscopes, together 

 with the apparatus which held them, he bequeathed to our Royal 

 Society. The greatest magnifying power amongst them has its 

 focus at -^q of an inch from the object, and is said to magnify 160 



