172 METHODS OF MICROSCOPICAL EESEARCH. 



effects. For instance, with a half-inch objective 

 and full aperture, carefully focus, on a ground glass 

 slip, the flame of the lamp, now interpose a dark 

 stop which should occupy, in the centre, about one- 

 third of the field ; removing the slip, replace ifc 

 with a group of (say) volvox-globator ; the plants 

 will be seen rolling from the outer ring of pure 

 transmitted light into the central black disc, where 

 they appear like emeralds; free-swimming rotifers 

 will pass backward and forward, from the outer 

 ring of parallel rays, into the eclipse of the dark 

 stop, where they become by oblique radiations, self- 

 luminous ; no finer example as showing in one 

 field, at the same moment, two extremes of illumina- 

 tion could be placed before the microscopical artist, 

 or an ordinary observer, to prove what may be 

 effected by an adept in the use of this beautiful in- 

 strument. With low powers striking presentations 

 of artistic illumination are under easy control; in 

 particular, the use of the paraboloid combined with 

 light from above. An experienced microscopist 

 is familiar with all these methods but the artist, 

 alert and eager for experimental conditions, often 

 hits upon effects not generally applied, possibly sac- 

 rificing scientific truth to aesthetic desire ; a result of 

 positions, and foci of illuminators their accurate 

 or eccentric adjustment, cutting off central or 

 peripheral rays, dispersing or half obscuring light 

 by intervening transparencies. The importance of 

 such combinations is paramount in the examination 

 of semi-opaque objects immersed in a thick bed of 

 medium, without pressure. These preparations are 

 in parts dense, even solid, combined with tissues of 

 extreme delicacy and transparency ; nothing being 



