54 



THE MICROSCOPE 



Cleansing 



cover 



''lasses. 



Tliickness 

 of cover 

 glass. 



Glass slip 

 with ledge. 



Blood films. 



much added to it. If the motion of the adjustment is ten divisions, 

 the true thickness is 15. 



It is essential that cover glasses before use should be thoroughly- 

 cleansed, and all specks, hairs, and fibres be removed. In most 

 cases a little soapy water will remove all dirt and grease, after 

 which they should be rinsed in clean water and dried with a clean 

 linen duster or chamois leather. Some microscopists use two 

 flat boards covered with chamois leather, between which the 

 cover glasses are rubbed, reversing the glass during the process 

 to make sure that it does not adhere to one board, thus cleaning 

 only one side. 



With low-power object glasses — IJ inch (32 mm.), 2/3 inch 

 (16 mm.), 1/3 inch (8 mm.) — the thickness of cover glass used is of 

 little importance ; but for high powers — 1/6 inch (4 mm.) or higher 

 power dry lenses — it is most important to always use the thinnest 

 covers (No. 1), because with high-power object glasses which are 

 not immersion lenses a variation in the thickness of the cover 

 glass affects the correction of the object glass. An object 

 glass can only give the most perfect image when used with a cover 

 glass of a particular thickness, and they are always adjusted for 

 the No. 1 cover glass (see page 81). The 1/6-inch (4-mm.) object 

 glass is very sensitive in this respect, and one apochromatic 

 lens of this power is provided with a correction collar to adjust 

 for cover glass of different thicknesses. As microscopic cover 

 glass is sold by weight, the cost of the No. 1 glass is not materially 

 more than the No. 2 or 3, because a larger number go to the 

 ounce. 



If a specimen in the nature of a leaf, a fibre, or powder, is to 

 be examined under a high power, it is best to place such a specimen 

 on a glass sUp and place a cover glass over it to flatten it out and 

 hold it in position, preferably in a drop of water. 



In this case a slip with a ledge against which the cover 

 glass may rest is a convenience. 



If the specimen is to be ex- 

 amined in fluid, a drop should 

 be placed on the slip and a cover 

 glass put down over it at an 

 angle in such a manner that the 

 cover glass touches one side of 

 the drop first, and is then allowed 

 to gradually fall so as 

 to prevent air bubbles 

 being enclosed (Fig. 49). 

 Blood films, or speci- 

 mens of bacteria which 

 are to be examined 

 and then destroyed, may be dried by heating over a spirit 

 lamp upon the slip or cover glass. If they are to be examined 



Fig. 48.— No. 3406, Slip with 

 Ledge. 



Fig. 49. 



