PRACTICAL MICROSCOPY. 



and by means of a pipette drop a little of the dilute alcohol 

 and diatoms upon it, applying a gentle heat with a spirit- 

 lamp. The alcohol takes fire and burns off. The remaining 

 alcohol causes the diatoms to become evenly distributed. 

 If inclined to mat, touch with a hot pin or needle. Now 

 bring the whole to a red heat for plenty of time to make 

 the diatoms appear white and perfectly clean. 



Now take a thin glass slip upon which a very thin cell of 

 brown varnish has been made and well aged as previously 

 described, take up the thin cover with diatoms attached and 

 place it upon the slightly gummed end of a cedar-wood 

 pencil, the diatomed side being uppermost. Now with a 

 fine sable pencil coat the edge of the thin cover with a 

 layer of thick brown varnish, wait until it has nearly set, and 

 invert it carefully and firmly upon the cell-walls of the slip. 

 A slight pressure with the pencil will cause the cover to 

 adhere all round, when the slide may be set aside for about 

 half an hour ; it must then be placed on the turntable and a 

 slight coat of brown varnish applied to the edge of the circle. 



The slide should now be put away for a week or more, 

 after which time it should be examined with the power 

 suitable to it and if not satisfactory, discarded. If, on the 

 other hand, it is a good preparation, the student may pro- 

 ceed to finish it by placing it on the turntable and describing 

 a ring of white zinc varnish so as to cover the edges of the 

 circle, making it completely air-tight. When this is dry, 

 various coloured rings may be turned upon the white sub- 

 stratum, giving the whole a very pleasing appearance. 



Some writers have deprecated this ornamentation of 

 slides, but the author's opinion is that the time is well spent 

 over a good slide, while a bad one, or even one of medium 

 quality, should be washed off as soon as examined for the 

 first time. The composition of the coloured varnishes may 

 be found in the list of recipes at the end of this chapter. 



