314 SLIDES AND COVER-GLASSES [Ch. X 



colorless glass slips used in America and so excellent for ordinary 

 microscopic work should be easily attacked by all liquids; even water 

 extracts a relatively enormous amount of alkalies and alkaline earths. 

 The slips of greenish glass, while not as neat or desirable for general 

 microscopy, seem to be decidedly more resistant, and are therefore 

 preferable." Transparent celluloid sUdes are recommended by 

 Behrens for work where hydrofluoric acid and its derivatives are to 

 be examined. (Chamot, Jour. Appl. Micr., vol. iii, p. 793. Chemical 

 Microscopy, p. 123-124). 



§ 487. Thickness of slides for special purposes. — It is very impor- 

 tant to observe strictly the requirements for the thickness of sUde for 

 special purposes. As pointed out in discussing the dark-ground con- 

 denser (§ 125-127), the slide must be thin enough so that the focus 

 of the condenser will be just above the upper surface where the object 

 is mounted. If the shde is too thick the focus will be beneath the 

 object and the best light cannot be obtained. So likewise with the 

 best achromatic condensers, especially when used as homogeneous 

 immersion condensers (§ 471), if the sHde is too thick the focus of the 

 condenser wiU fall below the object and the best and most critical 

 images cannot be obtained. 



It is better to use a slide thinner than the maximum permissible 

 and plenty of homogeneous liquid between the slide and the condenser, 

 then the condenser can be lowered until its focus is upon the object. 

 This applies equally with the dark-ground condenser. For getting the 

 thickness of the slides, use the micrometer calipers or the cover-glass 

 measurer (fig. 188-^189). 



§ 488. Cleaning slides for ordinary use. — Place new slides that 

 are to be wiped at once in a glass vessel of distilled water contain- 

 ing 5 % ammonia. For wiping the sHdes use a lintless towel or a 

 well-washed linen towel. One may avoid large wash bills by using 

 absorbent gauze (§ 488a). 



In handling the slides grasp them by the edges. Cover the fingers 

 of the right hand with the wiping towel or the gauze and rub both 

 faces with it. When wiped thoroughly dry, place the slide in a dry 

 glass jar or for larger numbers use a museum jar (fig. 214). . Soap and 

 water are also recommended for new sUdes. 



