'218 THE MICROSCOPE. 



drop it immediately over the object, slightly warm the 

 under part of the slide over a spirit-lamp to diffuse the 

 balsam and cause it thoroughly to penetrate the object, 

 and immediately cover the latter with one of the small 

 circles of thin glass, sold by opticians for the purpose. 

 In laying the glass cover on the object, care should be 

 taken to bring the edge of the circle down first, and let 

 the other fall slowly on the object (see fig. 139), to prevent 

 the formation of bubbles from the sudden displacement of 

 the air. It requires some little practice to keep the object 

 in the centre of the circle. 



Sometimes, notwithstanding all the care of the operator, 

 bubbles will appear. These may generally be got rid of 

 by gently warming the under part of the slide over the 

 spirit-lamp, when the bubbles will usually leave the 

 object, and travel towards the edge of the circle. In most 

 cases they will entirely disappear as the balsam becomes 

 firmer and drier. If it be desired to dry the balsam 

 quickly, the slide may be placed in some warm situation 

 where the heat does not much exceed 100°, and it must 

 be maintained in a perfectly horizontal position, to prevent 

 displacement, until the balsam has become dry. When 

 this has been ascertained to be the case, the superfluous 

 balsam which surrounds the edge of the circle may be 

 scraped off by the point of a penknife ; and when the 

 major part has been removed in this way, the remainder 

 may be got rid of, and the edges of balsam rendered smooth 

 by rubbing gently with an old silk handkerchief moistened 

 with spirit of turpentine. The edge of the circle of balsam 

 will probably appear white and dull, but it may be rendered 

 transparent by gently warming the under part of the slide 

 over a spirit-lamp, and again placing the object in a warm 

 room till the balsam has a second time become hard and 

 dry ; after which the name of the object should be written 

 with a small writing diamond at one end of the slide. 

 Some microscopists prefer to cover the slide with orna- 

 mental paper, which may be procured very cheaply. 



In covering the slides with paper, their edges need not 

 be ground, but may be rubbed with a fine file, which will 

 prevent the sharp glass from damaging the paper cover, 

 and cutting the fingers of the operator. The foregoing i3 



