316 CLEANING SLIDES AND COVER-GLASSES [Ch. X 



of oily substance must be removed. The slides should be placed in 

 the dichromate cleaning mixture (§ 497) one day or more, thoroughly 

 washed with clean water and then in distilled water, or in 50% to 

 75 % alcohol. They are taken from the water or alcohol and wiped 

 dry as needed. In wiping keep two or more layers of the absorbent 

 gauze over the fingers. Only one slide is wiped with each piece of 

 gauze. The surface to touch the slides should never have been 

 touched by the hands, for a minute amount of oily substance leaves a 

 stratum on the slide which causes the liquids used to heap up instead of 

 flowing out perfectly flat. That is, the slide is wet with difficulty 

 and the liquid instead of forming a film tends to assume the spheroidal 

 state. Sometimes new gauze or other cloth used may not be wholly 

 free from oily substance, or the soap was not wholly eliminated in 

 washing. Such wiping cloths will not make the slides ready for good 

 films. Some workers soak the gauze in sulphuric ether to remove the 

 last traces of oily substance. This is done more especially in cleaning 

 cover-glasses for films (see below). Burnett, p. 22, in speaking of blood 

 smears, says: "The slides should be thoroughly clean. Unused slides 

 may be cleaned in strong soap or 'gold dust' solution well rinsed 

 in water, then placed in alcohol from which they are wiped and 

 polished." 



As intimated above, the best way to tell when slides or covers are 

 free from a surface film is to drop some water on the surface and then 

 hold the slide or cover nearly vertical. If the surface is clean the 

 water will run over the slide, leaving a smooth wet track. If a film 

 of oily substance is present the water will crawl and form ridges or 

 droplets and not leave a smooth wet surface. Sometimes it is almost 

 impossible to get a slide so that a smooth, even film of blood or other 

 liquid can be made upon it. According to Chamot (p. 124), such slides 

 may be rendered suitable for use, in many cases, by passing them 

 slowly through the Bunsen flame. Cover-glasses are also rendered 

 usable by the same method when they are refractory after wiping 

 carefully. 



§ 491. Cover-glasses or covering glasses. — These are circular 

 or quadrangular pieces of thin glass used for covering and protecting 

 microscopic objects. They should be very thin, 0.1^ 1 > 0.^5 milli- 



