ON MOUNTING MICROSCOPICAL OBJECTS xxix 



series of sections the slide may be 1^ inch wide. The glass 

 should be free from air-spaces and other flaws, and the edges 

 should be ground. 



Cover-glasses must be very thin (No. 1.) and free from 

 flaws. For single objects circular covers ^ to | inch in dia- 

 meter are best, and for series of sections square or oblong 

 covers which should not come within -^ inch of the edge of 

 the slide ; space should also be left at one end for a label. 



Cells are required to protect thick objects from the pressure 

 of the cover-glass. Thin flat rings of tin are good, and should 

 be cemented to the slide with gold size or ' brown cement ' ; 

 or a ring of the cement painted on with a brush and allowed 

 to dry may be used as a cell for thinner objects. No cell is 

 required for sections. Copper, brass, and vulcanite cells are 

 to be avoided. 



An object may be mounted dry by fixing it to the slide in 

 the centre of a cell with a very small quantity of coUodion or 

 gum-water, or of solution of shellac in creasote or alcohol. 

 When the cement is dry, a cover of the same diameter as the 

 ceU is to be laid over it, and secured by varnishing at the 

 edge with any of the cements sold for the purpose. 



More usually it is advisable to mount an object in a liquid 

 or solid medium. Such are glycerin, which is liquid ; Par- 

 rant's medium, and Canada balsam, which are Kquids becom- 

 ing sohd ; and glycerin -jelly, which is solid when cold. 



An object, stained or otherwise, to be mounted in glycerin 

 or Farrant's medium may be transferred to it direct from 

 water — great care being taken to avoid introducing air. The 

 drop of glycerin, etc., used should be just large enough to 

 spread out to the edge of the cover. When glycerin is used, 

 the cover must be fixed by means of gold-size or other cement 

 •applied round the edge with a small paiat-brush. 



To mount in glycerin -jelly the object is first soaked in 

 glycerin and then in melted glycerin -jelly {see Appendix), it 

 is then transferred to a drop of the melted glycerin-jeUy upon 

 the sKde and the warm cover, previously wetted with the 

 melted jelly to aid in excluding air, is immediately placed 

 upon it and held in position till the jelly solidifies. Avoid 



