DEFINING AND PENETRATING POWER. 57 



the circulation in the frog, <fcc., the eye-piece a is best 

 adapted. 



For examining the details of any of the above objects, 

 it will be advisable to substitute the eye-piece b, which 

 also should be used in the observation of crystals when 

 illuminated by polarised light, the pollen of flowers, minute 

 dissection of insects, the vascular and cellular tissues of 

 plants, the Haversian canals and lacuna? of bone, and the 

 serrated lamina? of the crystalline lens in the eyes of birds 

 and fishes. 



The eye-piece c is of use when it is requisite to investi- 

 gate the intimate structure of delicate tissues; and also in 

 observations upon fossil infusoria, volvox, scales from 

 moths' wings, raphides, &c. The employment of this eye- 

 piece, when a higher power is required, obviates the neces- 

 sity of using a deeper object-glass, which always occasions 

 a fresh arrangement of the illumination and focus. It 

 must be borne in mind, that the more powerful the eye- 

 piece, the more apparent will the imperfections of the 

 object-glass become ; hence less confidence should be 

 placed in the observations made under a powerful eye- 

 piece than when a similar degree of amplification is 

 obtained with a shallow one and a deeper object-glass. 



The degree of perfection in the construction of the 

 optical part of a microscope is judged of by the distinct- 

 ness and comfort with which it exhibits certain objects, 

 the details of which can only be made visible by combi- 

 nations of lenses of high magnifying power, and a near 

 approach to correctness. Such are termed by the micro- 

 scopist test-objects. Mr. C. Brooke, F.R.S., whose labours 

 have been devoted to the correction of errors which have 

 crept into this part of philosophical research, says: — In 

 order to arrive at any satisfactory conclusions regarding 

 the action of any transparent medium on light, it is neces- 

 sary to form some definite conceptions regarding the ex- 

 ternal form and internal structure of the medium. This 

 observation appears to apply in full force to microscopic 

 test-objects; and for the purposes of the present inquiry, 

 it will suffice to limit our observations to the structure of 

 two well-known test-objects, — the scales of Podura plumbea, 

 and the siliceous loricae, or valves of the genus Pleurosigma t 



