ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 137 



completely obscures the light, and only admits the flash when it 

 retreats far enough in the other direction to uncover the side of the 

 diaphragm, then a slight shifting of the whole stroboscope will 

 lengthen the duration of the flashes without affecting their rate ; while 

 the rate can be varied by adjusting Bernstein's acoustic contact- 

 breaker, which regulates the vibrations. Moreover, by this method 

 the object can only be illuminated once in each complete oscillation 

 of the strip, while with a vibrating slit it may be illuminated either 

 once or twice. A frog's palate examined with this apparatus was 

 foimd to have a period of ciliary movement varying from ten to 

 fourteen (mostly from eleven to twelve) vibrations in a second. 



A second indirect means of measurement may be used as a check 

 upon the direct determination of the period from the phenomenon 

 already mentioned. When the rate of the stroboscope is equal to 

 that of the cilia, they appear as nearly as possible stationary ; as the 

 rate is increased waves of motion will be seen to run along them 

 until a point is reached at which they appear to be in uniform motion. 

 It will be found that at this point the rate of the instrument is 

 exactly double that of the cilia. 



Various rotifers examined by intermittent light showed the cilia 

 perfectly stationary. The ciliary filaments of some of the Infusoria 

 {Vorticella and Stentor), when viewed by intermittent light, not only 

 appeared to stand still, but their length seemed much greater than 

 with continuous 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.* 



Accessories for Microscopical Drawing.j — G. S. S. writes that 

 it often happens to him, when wishing to draw a mounted object, that 

 it is not placed exactly in the position in which it is wished to draw 

 it, and to so place it, the slide requires raising at one end or side. 

 For this purpose he devised a very simple piece of apparatus. 



Fig. 23. 



A piece of thin wood a little longer than an ordinary slide is cut, 

 and a hole 3/i in. square made in the middle. About 1/2 in. from 

 either end, and on the lower side, cut a narrower transverse groove, 

 and slip an india-rubber band over each end until it reaches the 

 groove. The slide to be examined is placed on the wooden one 



* Loc. cit. t Sci. -Gossip, 1886, p. 8 (2 figs.)- 



