138 C. I\ ROUSSELET OX THE ROUSSELET COMPRESSORIUM. 



4. The possibility of adding a drop of water to the secured 

 animal, or a drop of narcotising or fixing fluid, or other reagent 

 when desired, without risk of losing the animal and without 

 removing the compressor from under the microscope, or lifting 

 the cover-glass. 



All these points, which I consider most essential, are fully 

 secured by this compressorium, and all those who have tried to 

 follow and study a roving small animal with a high power will 

 appreciate the facility and precision with which this can be done 

 with the appliance here described. Some of the former com- 

 pressors, which were otherwise good in design, failed in so far 

 that they allowed the animals to wander under the brass ring 

 and out of sight, or to a position near the edge of the drop of 

 water where they could neither be reached with the high 

 powers of the microscope, nor properly illuminated from below. 



The accompanying figure shows the Rousselet compressorium 

 and renders a detailed description almost unnecessary. I will, 

 however, mention the dimensions of the parts and some essential 

 points which should not be overlooked by the makers, but most 

 carefully attended to in every detail in order to make the 

 apparatus thoroughly efficient. 



The length of the brass base-plate is 3| in., its width 1| in., 

 its thickness ^\ in. The glass tablet, well polished and free from 

 scratches, is T * F in. thick and § in. in diameter, and fixed almost 

 flush with the underside of the slide, which is slightly countersunk 

 on both sides ; when fixed, the tablet stands a very little (equal to 

 the thickness of a thin cover-glass) above the surface of the brass 

 slide. It will be noticed also that the glass tablet's position is 

 not in the centre of the slide, but a little above it. 



The arm carrying the cover-glass must be a little stouter than 

 the slide, say ^ in., in order to secure sufficient rigidity; it forms 

 a ring of 1] in. outer and g in. inner diameter, about one-third 

 of which is cut off on the upper side. The ring is bevelled from 



the outer to the inner edge. 



