INT ROD UCTION. 



XXV 



METHOD OF CLEANING COVER-GLASSES. 



Not unfrequcntly covers are coated with a dull film, which obscures the passage of the 

 light. To remove this film place the cover-glasses for an hour or two in a beaker containing 

 strong sulphuric or nitric acid. Pour off the acid, and wash the glasses thoroughly with 

 water until all the acid is removed. They may then be placed in methylated spirit, and as 

 each cover-glass is taken out it is cleaned with chamois leather or a well-washed silk hand- 

 kerchief. This is accomplished by rubbing the surfaces of the cover-glass with the hand- 

 kerchief, held between the thumb and forefinger of the left hand. The cover ought always to 

 be lifted by the edges, and never allowed to lie on the flat after it is cleaned, as particles of dirt 

 are apt to adhere to it. Tilt it up against something until required. For inch-and-a-quarter 

 cover-glasses a good plan is to have two perfectly smooth pieces of wood, covered with 

 chamois leather, and to place the cover-glass between them, and rub it until cleaned. 



ON MEASURING COVER-GLASSES. 



As already indicated, No. I cover-glasses do very well for the ordinary purposes of a 

 student. When very high powers are used, it is well to know the thickness of the cover-glass 

 employed. Cover-glasses may be rapidly measured in two ways. 



(a) Micrometer caliper. This little American instrument (fig. 8) is sold by Messrs. Chas. 

 Churchill and Co., 28, Wilson Street, London. It is used for measuring the thickness of fine 

 brass plates. The piece in the form of the letter 

 U has a projecting hub, , at one end. Through 

 the two ends are tapped holes, in one of which 

 is the adjusting screw, B, and in the other the 

 gauge screw, C. Attached to the screw, C, is 

 a thimble, D, which fits over the exterior of the 

 hub, <7. The end of this thimble is bevelled, and 



Fig. 8. POCKET SHEET-METAL GAUGE FOR MEASURING 

 COVER-GLASSES. Natural Size. 



Fig. 9. COVER-GLASS MEASURER OF ZEISS. 



the bevelled edge graduated into twenty-five parts, and figured o, 5, to, 15, 20. A line of 

 graduation, 40 to the inch, is also made upon the outside of the hub, a, the line of these 

 divisions running parallel with the centre of the screw, C, whilst the graduations on the 

 thimble are circular. The pitch of the screw, C, being 40 to the inch, one revolution of the 

 thimble opens the guage i-4Oth or 25-ioooths of an inch. The divisions on the thimble are 

 then read off for any additional part of a revolution of the thimble, and the number of such 

 divisions are added to the turn or turns already made by the thimble, allowing 25-1000 for 

 each graduation on the hub, a. 



