21 



THE ART OF MAKING MICRORCOrE SLIDES 



Sealing 



Sealing the Coverslip in Place 



It was pointed out in the last cliapter, 

 and must be reiterated here, that before 

 any sealing cement can be applied a pro- 

 tective barrier must be erected to prevent 

 this cement from running in by capillary 

 attraction and mixing with the contents 

 of the cell. More wholemounts are spoiled 

 by this running in of cement than by any 

 other method. The procedure, when 

 mounting on a flat slide, or one containing 

 a concavity, is somewhat different from 

 that which is used in sealing a cell. The 

 former will be described first. 



The process of attaching a coverslip 

 and sealing it in place on an aqueous fluid 

 mount in a concave shde is shown in sec- 

 tion in Figs. 9-14. Fig. 9 shows a longi- 

 tudinal section of the concave slide with 

 the protective ring of cement in place. 

 This ring should be the narrowest and 

 the thickest which can be made with the 

 aid of the finest sable-hair brush. It 

 should certainly not be wider than >^4 of 

 an inch, and if it can be built up to twice 

 this depth, it will be all the better. This 

 ring also assists in attaching the coverslip, 

 so that the gold size should be given time 

 to become tacky before the mount is 

 made. It is a matter of convenience to 

 run such rings on the turntable on the night 

 before the mount is to be made. A ring 

 made from a good specimen of gold size 

 will remain tacky for at least 48 hours 

 after it is turned. The diameter of this 

 ring is quite critical. It should be at the 

 outer edge about J-^4 of an inch less than 

 the diameter of the coverslip. If it is 

 smaller than this, too big a space will be 

 left between the edge of the cell and the 

 coverslip; if it is larger than tliis, perfect 

 sealing is impossible. 



Fig. 10 shows the same slide after the 

 object has been placed in the cavity and a 

 sufficient quantity of the selected mount- 

 ing medium placed on top of it. Notice 

 that a great excess of the medium is pro- 

 vided and permitted to rise up in a convex 

 meniscus. After this drop has been placed 

 in position the mount should be inspected 

 carefully to make quite certain that the 

 fluid is in contact with the protective 

 ring of varnish all the way around the 

 edge. If the slide is perfectly clean it 



may so happen that the meniscus does 

 not extend to the varnish ring, leaving a 

 small air gap which will result in a bub- 

 ble — almost impossible to remove sub- 

 sequently. The next step is that of placing 

 the coverslip in position. The covershp 

 must never be let down from one side in 

 the manner customarily taught in making 

 balsam mounts. It must be held between 

 the thumb and second finger and lowered 

 horizontally until it is in the position 

 shown in Fig. 11. It will be seen that the 

 object remains in the central position in 

 which it started whereas, if the cover were 

 lowered from the side, the object would 

 inevitably be pulled by capillary attrac- 

 tion to one corner whence it would be 

 almost impossible to displace it. 



Fig. 12 shows that the covershp has 

 been let down and pressed with a needle 

 onto the surface of the tacky protective 

 ring of gold size. The excess fluid has 

 been pushed out and mopped up with a 

 filter paper. Care should be taken to 

 remove the whole of the fluid between the 

 outer projecting edge of the covershp 

 and the ring to which it is attached. This 

 is one of the most critical stages in the 

 whole procedure. The needle used to press 

 the covershp in place should be run with a 

 circular movement round the covershp 

 vertically above the protective ring, and 

 pressure should be continued until the 

 glass is clearly in contact with the gold 

 size at all points. The shde is then placed 

 on the turntable, centered, and a ring of 

 the selected second cement applied round 

 the edge. 



The result of this is shown in Fig. 13, 

 where it will be seen that the edge of the 

 coverslip is firmly embedded in the cement 

 which has run under as far as the pro- 

 tective barrier. The existence of the pro- 

 tective barrier and the overhang of the 

 covershp insure, therefore, that there 

 shall be a good, thick layer of this cement 

 in position. The shde is now laid on one 

 side until this first protective layer is 

 thoroughly dry and then (Fig. 14) as 

 many rings of asphalt varnish turned over 

 the top as are required. It is an excellent 

 thing to apply two coats of asphalt var- 

 nish, naturally permitting the first to 

 dry before applying the second, at the 



