THE SPONGIDA OR PORIFERA. 341 



blotting-paper, constitutes a practically air-tight chamber. Every- 

 thing is now in working action, and it only remains to add the 

 material to be examined, and to fill the reservoir with water. The 

 water from the reservoir soaking through the bibulous paper, 

 keeps the air-tight chamber constantly moist, and evaporating 

 faster from its contained free circular edge, prevents loss of 

 moisture from beneath the covering-glass. The water in the 

 reservoir will maintain the moist chamber in the above conditions 

 for many days, and will require replenishing only at distant 

 intervals." If continuous observation is not requisite, and the 

 microscope is needed for other purposes, the slides may be trans- 

 ferred to an " ordinary moist chamber, which may be extem- 

 porised out of a tumbler, or small bell glass, inverted upon a 

 plate containing a few folds of well-saturated bibulous paper." 



For " obtaining the most satisfactory illuminations and defi- 

 nition of minute flagellate organisms," Saville Kent adopted a plan 

 recommended by E. M. Nelson, F.R.M.S. This necessitates a 

 horizontal position for the microscope. " The mirror being turned 

 to one side, the microscope and lamp are so disposed that the 

 central ray of light from the 7iarrow edge of the lamp flame passes 

 through the optical axis of the condenser, and is then focussed 

 upon the field of view, by means of the substage rackwork, in such 

 a manner that, employing a i-inch object glass, a sharply defined 

 image of the lamp flame, edge on, is projected upon the centre of 

 the field in company with the objects under examination. If the 

 T-inch object glass is now detached, and a T/i6th, i/25th, or i/5oth 

 substituted, and focussed into place, a slight re-adjustment of the 

 centering of the achromatic condenser being perhaps required, it 

 will be found that the entire field is brilliantly illuminated, and the 

 most minute objects defined with an amount of sharpness rarely 

 obtained under other conditions. In addition to the ordinary 

 graduating diaphragm, "the interposition of a second diaphragm 

 at the lowest point in the substage arrangement" is of considerable 

 advantage. 



We have quoted this method, as Saville Kent found it so useful 

 in examining minute infusorial forms with high-power objectives 

 ranging from i/i6th to i/5oth. Good work can be done, however, 

 by means of a i/i2th inch object glass with an Abbe's condenser. 



