58 



mixture whose formula is given below may be used. Place the cover-glasses in 

 this mixture, one by one, and permit them to remain over night or longer. Rinse 

 them thoroughly in running water until all color of the dichromate has disappeared 

 rinse them again in distilled water and transfer to 95% alcohol, out of which they 

 may be wiped. 



170. Dichromate Cleaning Mixture. Formula: Potassium dichromate, 

 200 grams; water, 800 c. c.; strong sulphuric acid, 1200 c. c. Dissolve the 

 dichromate in the water by the aid of heat, and to the solution add slowly the 

 sulphuric acid. The two fluids should be mixed in a lead-lined kettle [17]. 



171. Used slides and cover-glasses, vials and other glassware that have 

 been used with balsam, cedarwood oil, or other oily substance, etc., may be 

 cleaned by boiling them with a solution of strong soap, such as "gold dust," one 

 or more changes. Used xylene or toluene is sometimes useful. Slides and covers 

 may require a second cleaning with the cleaning mixture. If only water, glycerin 

 or glycerin- jelly has been used on them, they may be cleaned with water, prefer- 

 ably warm water, and then, if necessary, wiped out of 50% alcohol. 



172. Measuring the thickness of the cover-glasses. With the cover- 

 glass measurer determine the thickness of the cover-glasses and sort them into 

 three groups: (a) those with a thickness of .13-.17 mm., (b) those less than 

 .13 mm., and (c] those thicker than .17 mm. Groups (a) and (b) only should be 

 used; (c) should be discarded or used only with objects for low magnification. 



It is advantageous to know the thickness of the cover-glass on an object 

 for the following reasons: (a) That one do not try to use objectives in studying 

 the preparation of a shorter working distance than the thickness of the cover- 

 glass [17] ; (b) In using adjustable objectives with the collar graduated for different 

 thicknesses of cover, the collar might be set at a favorable point without loss of 

 time; (c) For unadjustable objectives the thickness of cover may be selected 

 corresponding to that for which the objective was corrected [17]. Furthermore if 

 there is a variation from the standard one may remedy it in part at least by 

 lengthening the tube if the cover is thinner and shortening it if the cover is thicker 

 than the standard [17]. 



THE MICROTOME KNIFE. 



Finally, the microtome knife or section razor should receive a 

 passing word as upon it depends far more than may be at first sus- 

 pected the excellence of results. Scrupulous care should be taken 

 to maintain a keen edge, smooth and free from nicks or corrosion. 

 Never touch the edge with anything hard or metallic. Keep it clean. 

 Before and after using it is advisable to strop it upon a strop backed 

 with wood, and occasionally it will be necessary to hone it. 



173. For microtome knives two grades of hones are service- 

 able; the yellow Belgian for first sharpening and either a blue-green 

 water hone or a fine Arkansas oil stone for finishing. These should be 

 kept clean and free from dust. If a section knife is used a great 



