HARDWICKE'S SCIENCE-GOSSIP. 



85 



filled with brine, and made to impinge on a small 

 plano-convex lens attached to the base of the instru- 

 ment, the concentrated beam falling on the object. 

 Hooue thus describes his method of measuring the 

 magnifying power of his microscope :—" Having 

 rectified the microscope to see the desired object 

 through it very distinctly, at the same time that 



Fig. 4S. Lieberkuhn's simple Microscope : a, short tube ; b, 

 convex lens for condensing the light en the speculum c, 

 from thence reflected on the injection d; e, eye lens for 

 viewing objects. 



I look upon other objects at the same distance 

 with my other bare eye, by which means 1 am able 

 by the help of a ruler divided into an inch and 

 small parts, and laid on the pedestal of the micro- 



Fig. 46. Central portion of Ocular of Martin's Microscope. 



scope, to cast as it were the magnified appearance 

 of the object upon the ruler, and thereby exactly 

 measure the diameter it appears of through the 

 glass, which being compared with the diameter 

 it appears to the naked eye, will easily afford the 

 quantity of its magnifying." 



To Hooke may justly be ascribed the invention of 

 spherical lenses of high power. The following is his 

 description of his method of making them :— " If you 

 take a clear piece of Venice glass, and in a lamp 



draw it out into fine threads, and then holding the 

 ends of these threads in the flame until they melt, 

 they will run into a small round globule or drop, 

 which will hang to the end of the thread. Having 

 made a number of these, they are all to be stuck 

 upon the end of a stick, with the threads standing 

 uppermost ; these ends are to be ground off fine on 

 a whetstone, and then polished on a metal plate 

 with tripoli. The lenses thus finished, if placed 

 against a small hole made in a thin piece of metal 

 and fixed there with wax, will both magnify and 

 make some objects more distinct than any great 

 microscopes can do." 



Whilst Hooke was pursuing his microscopic 

 investigations in England, Eustachio Divini, of 

 Rome, and S. Campani, of Bologna, were also 

 engaged in endeavouring to' improve the micro- 

 scope, and making observations with it. The former 

 published a description of his instrument in the 



Fig. 47- Diagram of Micrometer in Martin's Microscope: 

 a, eye-lens; b, field-lens ; c, screw; d, index. 



"Philosophical Transactions" for 1868; it differed 

 from Hooke's in several particulars. Instead of a 

 double convex eye-glass, he substituted two plano- 

 convex tenses, which touched each other in centre of 

 their convex surfaces. The advantage of this con- 

 struction was twofold,— a flat field and great 

 magnifying power. Chevalier, in his work on 

 "Microscopes and the Method of using them," says 

 that the compound body of this instrument when 

 closed was 16 inches long, and as large in circum- 

 ference as a man's thigh, and that the eye-glass was 

 equal in size to the palm of the hand. 



Philip Bonnani, in 1691, published his " Observa- 

 tiones circa viventia, quae in rebus non viventibus re 

 periuntur, cum micrographia curiosa,"a quarto, con- 

 taining 445 pages and 68 plates. This is the work 

 to which Leeuwenhoek alludes in his " Refutation of 

 the Doctrine of Equivocal or Spontaneous Genera- 

 tion." In the following year, Bonnani published 

 his "Micrographia curiosa, sive rerum minutissi- 

 marum Observationes quae ope microscopii recognitor 

 et expressse describuntur," 4to, pp. 106, plates 40. 

 In the former work he describes his microscope. 



