280 The Microscope. 



studied. To apply intermittent light to the microscopical ex- 

 amination of ciliated organisms, the writer has* devised an elec- 

 trically rotated aperture disk, which is arranged to interrupt 

 the beam of light employed in illuminating the object to be 

 examined. 



The instrument consists of an electric motor of the sim- 

 plest kind mounted on a plate having a collar fitted to the sub- 

 stage of the microscope. The shaft, which carries a simple bar 

 armature before the poles of the magnet, also carries upon its 

 upper extremity a disk having two or four apertures, which co- 

 incide with the apertures of the stage and substage two or four 

 times during the revolutions of the disk. 



The shaft carries a commutator, and the course of the cur- 

 rent from the battery through the instrument is through the 

 spring touching the commutator, through the shaft and frame of 

 the instrument to the magnet, thence out and back to the bat- 

 tery. There are two methods by which the speed of rotation of 

 the aperture disk may be varied; one is by plunging the ele- 

 ments of the battery more or less, and the other is by applying 

 the finger to the shaft of the motor as a brake, the motor in the 

 latter case being started at its maximum speed, and then 

 slowed down to the required degree by the friction of the fin- 

 ger. Experiment shows that the period of darkness should be 

 to the period of illumination about as three to one for the best 

 effects. Closing two diametrically opposite holes in the disk 

 secures about the correct proportion. 



Various rotifers examined by intermittent light showed the 

 cilia perfectly stationary. The ciliary filaments of some of the 

 infusoria, Vorticella and the Stentor, for example, when 

 viewed by intermittent light, appeared to stand still, and their 

 length seemed much greater than when examined by continu- 

 ous light. The interrupted light brings out not only the cilia 

 around the oral aperture, but shows to good advantage the cilia 

 disposed along the margin of the body. What interrupted light 

 may reveal in the examination of flagellate or ciliated plants 

 the writer is unable to say, as no objects of this character have 

 been available. It is presumable, however, that something in- 

 teresting will result from the examination of Volvox and other 

 motile plants, by means of this kind of illumination. Although 



