142 MrcRoscopic illustrations. 



Ross, who first proposed the adoption of this reflector, 

 places a condensing lens in front of the stage. The dis- 

 persion occasioned by this lens, however, is apt to give 

 a false colouring to the object ; so that I prefer 

 making use of a large concave mirror in the manner 

 represented in the above (fig. 31), which obviates this 

 defect, whilst it admits of the light being placed in a 

 more convenient position. 



Sect. 19. A Method for determining the Forms of certain 

 minute Bodies. — In the investigation of long stick-shaped 

 bodies, it is sometimes difficult to ascertain Avhether a 

 section of these would be of a circular, or of a flattened 

 oval form ; since, in either case, the object, when at rest, 

 presents the same appearance to the eye of the observer. 

 It occurred to me, therefore, that by immersing a body 

 such as these in a fluid, which evaporates rapidly, the 

 currents occasioned by the evaporation would raise and 

 turn it upon its edges, so as to afford a view of its different 

 sides. This plan I found to answer my expectation fully. 

 As an example illustrative of this mode of verification 

 may not be without its use, I have thought it worth while 

 to present the reader with the following one : — 



Take on the point of a penknife a specimen of fossil 

 infusoria (or more correctly speaking, the silicious cover- 

 ing of these creatures), place it on a slip of glass, or what 

 is perhaps more convenient, the tablet of an aquatic live- 

 box, the cover being removed. Having equipped your 

 microscope with a moderate power, and .adjusted it to 



