C. F. ROUSSELET ON THE ROUSSELET COMPRESSORIUM. 139 



The cover-glass, 1J in. to 1| in. in diameter and with a segment 

 cut off, must be cemented firmly to the perfectly flat (not counter- 

 sunk) under-surface of the ring with a very little goldsize. This 

 is best applied to the brass ring and allowed to dry for half an 

 hour, the cover being then put into proper position and very 

 gently warmed, when it will adhere firmly on cooling and be 

 ready for use. The cementing of the cover-glass is most important, 

 in order to prevent the fluid from a water or oil immersion objec- 

 tive running between the brass ring and cover-glass, and ulti- 

 mately finding its way to, and mixing with the water on the 

 slide. In cementing, the cover-glass should be so arranged that 

 the cut edge just reaches the upper edge of the glass tablet ; this 

 arrangement will allow a drop of water or reagent to be added 

 with a fine pipette while the animals are under examination. 

 The thickness of the cover-glass may be anything between 0*16 

 and 0*20 mm., but the thickness should always be known. 



The arm moves parallel to the glass tablet, and is raised and 

 lowered by a screw adjustment working against a spiral spring 

 contained in the small brass drum. It is held in position by a 

 spring catch, but can easily be turned aside. Although the arm 

 is made to move parallel to the brass tablet, it should be to a 

 very slight extent inclined forward, so that on being lowered the 

 cover-glass first touches the distal part of the glass tablet, other- 

 wise it will be found that very small creatures cannot always 

 be firmly held. It is obvious also that if the cover-glass should 

 first touch the lower or the near edge of the tablet, no amount 

 of pressure by the screw will secure the animal ; on the contrary, 

 the other side of the cover will be raised. This condition of 

 things, when it exists, can be remedied by fixing a strip of paper 

 or thin glass to the under-surface of the ring at its distal part 

 before cementing the cover-glass. 



This compressorium was first show T n by me at the meeting of 

 the Royal Microscopical Society on April 19th, 1893, and was 

 figured and described in the journal of that society for the same 

 year (p. 386). I have had it in constant use ever since, and, 

 so far, have not found occasion to modify it or to suggest any 

 improvement. 



There is only one desirable quality that it does not possess, and 

 which cannot be given to it : it is not reversible. In actual 

 practice, however, I find that by raising the cover slightly whilst 



