Estimate oft/ie relative Value of' Microscopes. 3^ 



foiup eggs from the embryo state to the exclusion of the young. 

 But this fourfold increase in the space of a day, when no ob- 

 stacle intervenes, and the same individual gives, in ten days, 

 forty eggs, and raised to the tenth power (therefore on the 10th 

 day) a million of individuals from one mother; and on the 11th 

 day four, on the l^h sixteen, millions, &c. Although this produc- 

 tive power is the greatest which has been yetjobserved in nature, 

 far exceeding tliat of insects, it is far from attaining that of the 

 Polygastrica. In the Paramcecium aurelia, which is ^^"' in size, 

 and which has been ascertained to live several days, a doubling of 

 each individual by transverse division has been observed within 

 twenty-four hours, its rate of increase is therefore double that of 

 the preceding. But as these animals, besides division, also propa- 

 gate by eggs, and these eggs are not separated from the parent 

 singly but in masses, and as they also form gems, the possible 

 increase within forty-eight hours becomes quite innumerable. 

 Who can wonder that, under such circumstances, fluids should, 

 with the brood of two or three days only, swarm with these ani- 

 malcules P 



. Estimate of the relative Value of the Microscopes of Chevalier^ 

 Plpessel, and Schick. — The following are my individual views re- 

 garding the microscopes of Chevalier, Ploessel, and Schick, which 

 are the best of the present day. The advantages of all these 

 instruments rest upon the discovery of Selligue, and the most 

 important advances upon this basis have been certainly made by 

 Chevalier. 



The chief practical advantages of the microscope of Chevalier 

 are, — extent of field of vision, distinctness of outline, even with 

 the highest powers, high magnifying power, a sufficient distance 

 of the object-lens from the object, which amounts to a line eyen 

 whh the highest powers, simplicity of apparatus, and, conse- 

 quently, a very moderate price. .. ^^ V^ *' - 

 Ploessers microscope, compared witn that of Chevalier, is dis^' 

 tinguished by the following properties. With as extensiviea 

 field of vision, it has still greater, pjerhaps top great, light,^i( 

 m^^nifies nearly double the diameter with great distinctness of 

 o|4tline ; its use is more convenient under high powers, from the 

 ejpployrnent of sc\cral eye-glasses, instead of the elongation of 



