442 PKEPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



allows the outer thimble to slip. The connection of the spindle to 

 the measuring wheels is effected by means of a stop. This takes 

 into a slot on a sleeve, on which is mounted the thousandths wheel, 

 which in turn drives the hundredths and tenths wheels through the 

 intermediate pinions. These latter have a step-by-step motion, as 

 in an ordinary counter. The cover of the cage in which the 

 mechanism is placed is pierced to show the numbers on the dials, but 

 these openings are covered with glass, with a view to excluding dust 

 and dirt. It must be understood that gauges of this kind are 

 expensive, but there is one made by G. Boley, reading to '01, which 

 answers all purposes and can be purchased for five shillings at a 

 watchmaker's tool shop. 



It is well to keep assorted, measured, and cleaned cover-glasses in 

 small separate wide -stoppered bottles of methylated spirit, each 

 bottle being labelled with the gauge of thickness of the covers it 

 contains. What is then required is a simple apparatus for cleaning 

 the delicate covers with the least risk of breakage. This can be 

 well accomplished by having two blocks of boxwood, shaped so as to 

 be easily held one in each hand, turned with perfect trueness on the 

 faces opposite to the respective handles, so that when the surfaces 

 so flattened are laid upon and pressed towards each other they are 

 every where in perfect contact. They should be from two to four 

 inches in diameter, and these flattened surfaces should each have, 

 very tightly stretched upon them, a firm, even-textured, moderately 

 thick piece of chamois leather. If covers be slightly moistened 

 even breathed upon and laid on one of these blocks and pressed 

 down with the other, breath, or moisture applied by a small camel- 

 hair brush to the upper surface of the cover, may be applied, and a 

 few twists of these blocks upon each other when firmly pressed 

 together will effectually clean without breaking the thinner covers. 

 It will be often needful to treat both sides of the covers thus, as one 

 side generally adheres while the other is subject to the friction. 



For cleaning slips and covers by hand, finishing should be done 

 with old fine cambric handkerchiefs. These should not be washed 

 with soap, but with common soda and hot water, plenty of the latter 

 being subsequently employed to get rid of every trace of the alkali. 

 But when dry these cloths must not be ' ironed ' or smoothed in any 

 way, the ' rough-dry ' surface acting admirably for wiping delicate 

 glass. 



Varnishes and Cements. There are three very distinct purposes 

 for which cements which possess the power of holding firmly to glass, 

 and of resisting not merely water but other preservative liquids, 

 are required by the microscopist, these being (1) the attachment of 

 the glass covers to the slides or cells containing the object, (2) the 

 formation of thin ' cells ' of cement only, and (3) the attachment of 

 the * glass plate ' or ' tube-cells ' to the slides. The two former of 

 these purposes are answered by liquid cements or varnishes, which 

 may be applied without heat ; the last requires a solid cement of 

 greater tenacity, which can only be used in the melted state. Among 

 the many such cements that have been recommended by different 

 workers, two or three will be selected by the worker for general 



