THE MICROSCOPE AND ITS PARTS. 35 



regulated, and, with ordinary care, not flashing into 

 the tube. It is entirely successful. 



Evaporation of 'the water will take place from be- 

 neath the thin cover, sometimes rapidly, and the ob- 

 server will at first be surprised at the way in which his 

 objects will be swept out of the field before an advanc- 

 ing wave that leaves the glass nearly dry behind it. 

 The water in the cell is drying up, and a fresh supply 

 must be added if the objects are not to be entirely lost. 

 Here is another advantage in using square covers on 

 circular cells. The four corners project beyond the 

 cement ring, and by applying the camel's-hair brush, 

 wet with water, to the slide beneath any one of these 

 projections, the drop will run in and fill the cell by 

 capillary attraction. This supply is much more easily 

 added than if circular covers are used, and after a 

 little experience the fresh drops can be applied while 

 the eye is at the eye-piece, the hand alone guiding the 

 wet pencil, and the eye taking note of the rush of the 

 incoming wave and of the effect. The student will 

 soon become s.uch an adept that he will be able 

 to add so small a supply at each touch of the wet brush 

 that the movement of the capillary wave will not be 

 strong enough to float the object out of the field. 



But it often happens that a certain specimen is to be 

 studied for a long time, a whole evening, for instance; 

 but to be continually supplying the water lost by evap- 

 oration is not convenient — the student often becoming 

 so absorbed that he forgets this one of Nature's laws 

 until he suffers the penalty, and probaDly loses his 

 object. At such a time an arrangement is wanted for 

 supplying fresh water continuously and without de- 

 manding much attention, and such a contrivance is 



