EXAMINATION OF TEST-OBJECTS. 63 



formosum, F. quadratum, P. angulatum, and tlieir allies, 

 bring JSTo. 11 aperture in the field, rack up the con- 

 denser until the field is very bright, then put on No. 1 

 stop, and rack up the condenser until the stop disappears : 

 if the desired effect be not obtained, try jN'o. 2 stop ; the 

 condenser will then require to be racked up still higher, 

 and the dots will come out easily. To examine Navicula 

 cuspidatum, iV. rhomboides, Pleurosigma fasciola, Pluero- 

 sigma macrum, &c., still use No. 11 aperture, and stop 

 No. 2, which will require a slight alteration in position 

 only, when the checks will appear distinctly. 



" For the Amician test use the slots instead of No. 2 

 stop ; and for very difficult lines, such as those of the 

 Acus, the condenser must be so arranged (when focussed 

 up till the field becomes exceedingly bright) that a shadow 

 is thrown on the object from the left hand, and No. 1 

 and No. 2 stop used, so as to darken a little the right 

 hand side of the field. The -^th and j^th object-glasses 

 are generally found the best suited for the Acus with the 

 B eye-piece ; in fact, the B eye-piece is mostly to be pre- 

 ferred with high powers." 



In short, the whole requires perfect centering as well 

 as careful illumination, and the condenser used must 

 have a wide aperture : we have found Collins's answer 

 exceedingly well, and with it we have brought out the 

 markings on the P. fasciola, X. Acus, &g. 



When employing high powers for viewing test, or 

 indeed any objects, only that part which is exactly in 

 focus can be perfectly seen at one time ; and even a very 

 slight turn of the^we adjustment, or *' slow motion," often 

 causes a totally different set of appearances to present 

 themselves. It is desirable during our examination to 

 keep the finger on the slow motion, and thus bring into 

 view different parts or thicknesses of an object, so that in 

 the most careful manner layers or sets of vessels may be 

 seen and continuously traced. A clearer idea of the 

 nature of a doubtful structure is in this way obtained, 

 while we are in the act, as it were, of changing the focus. 

 When different structural features present themselves on 

 different planes, it is often a most difficult task to deter- 

 mine which of them is the nearer. This may be regarded 



