MICROSCOPIC CHRO^OPHOTOGRAPHY 297 



M. Xachet has constructed a condenser in the form of 

 a cone with a spheroidal base. This base is hollowed 

 out in the middle for the reception of a capsule con- 

 taining thick black varnish. If the light is thrown 

 on the apex of this cone, it will be reflected in the 

 form of converging rays, and illuminate the objects 

 contained in the preparation, Avhich consequently 

 stand out clearly against the dark central background. 

 But this arrangement is only applicable when the 

 magnifying power is low, and has not up to the 

 present produced any very interesting results. Yet 

 we do not desjDair of obtaining by some other means 

 a good series of photographs on a dark background, 

 and with fixed plates. 



Chronophotography on moving plates is of far easier 

 application, and the first object submitted by us to 

 this method of observation was a vortieella in active 

 movement. This species of infusoria is shaped some- 

 thing like a funnel, and is beset with a crown of 

 vibrating cilia. It is supported on a spirally twisted 

 stalk which is attached by its other extremity to 

 some vegetable fibre, and furnishes a fixed point of 

 support. 



From time to time the stalk executes sudden re- 

 tractions by approximating the turns of the spiral, 

 and the funnel-shaped bell is suddenly brought up to 

 the point of attachment. The stalk then lengthens 

 out, and the spiral arrangement disappears until the 

 following retraction. 



Fig. 201 shows, under a considerable degree of 

 magnification, several of these vorticellae in the midst 

 of a tangle of conferva filaments. 



To follow the movement more easily it is as well to 

 take as a guide some fixed point such as the intersec- 

 tion of two filaments selected from the network in the 

 field of the microscope. For instance, in the figure 



