114 



The Preparation of Microscope Slides 



Fig. 80. Transferring objects between reagents with cloth-bottomed tubes. 



alcohol. The author has long since abandoned tubes in favor of the device 

 seen in Fig. 80. This is a short length of glass tube, open at both ends, 

 which has a small piece of bolting silk or other fine cloth tied across the 

 lower end. The specimens are placed in the tubes which, as may be seen 

 in the illustration, are transferred from one stender dish to another with 

 a minimum chance of contamination. These tubes are commercially avail- 

 able in England, but in America they must be imported or homemade. 



There is no means of judging when dehydration is complete save to 

 attempt to clear the object. It is unwise to believe the label on an open 

 bottle or jar if it says "absolute alcohol" because this reagent is hygro- 

 scopic and rapidly absorbs water in the air. A quantity of anhydrous 

 copper sulfate should therefore be kept at the bottom of the absolute alco- 

 hol bottle and the alcohol no longer regarded as "absolute" when the salt 

 starts turning from white to blue. More wholemounts are ruined by being 

 imperfectly dehydrated than by any other method. Even the smallest 

 specimen should have at least 24 hr in absolute alcohol before any at- 

 tempt is made to clear it. 



Choice of a Clearing Agent. A clearing agent must be both miscible 

 with absolute alcohol and with the resinous medium that has been se- 

 lected for mounting. The ideal substances for this purpose are essential 

 oils. They impart just as much transparency to the specimen as does the 

 resin used for mounting, so that they provide, as it were, a preview of the 

 finished specimen. The use of benzene, which is recommended for prep- 

 aration of paraffin sections, has started to spread into the preparations 

 of wholemounts. In the author's opinion, it is utterly worthless for this 

 purpose. It has a relatively low index of refraction, so that it is impossible 

 to determine whether or not the slight cloudiness of the specimen is the 



