134 The Microscope. 



In cutting, it is of the greatest importance to have the blade of 

 the knife perfectly clean ; I am in the habit therefore of washing 

 my microtome knife in boiling water, every time after using it for 

 cutting paraffine, and then to wipe it off carefully with alcohol ; if 

 this is done conscientiously, there is little risk of the surface of the 

 knife getting soiled by the paraffine, and therefore very little risk 

 of any of the sections sticking to the blade of the knife and inter- 

 fering with the smooth slipping along of the ribbon of sections. As 

 regards the imbedding of the object, paraffine is the most useful 

 material, of course. I find a mixture of seven parts of hard par- 

 affine with twenty parts of soft paraffine the best. In warm weather 

 a little hard paraffine may be added, and in very cold weather it 

 may possibly be necessary to make a trifling addition of soft paraffine. 

 The paraffine is kept in a bath at a temperature of 57° to 59° centi- 

 grade. I consider it extremely important that the paraffine should 

 be filtered. For this purpose I use a hot water filter of the type so 

 well known in the chemical laboratory. The filter itself is a double 

 case of copper, which may be filled with water ; inside is a copper 

 funnel; below the filter is placed a ring burner by which the water 

 in the outer case is heated; a pipe runs from one side of the funnel 

 and is connected with a large Mariotti's flask, so that when the funnel 

 is heated and there is loss by evaporation the supply is kept up for a 

 long time at a constant level. It may be remarked that it is desir- 

 able to use rubber stoppers for the Mariotti's flask, as it is very 

 difficult to make corks fit air-tight. The inner funnel is furnished with 

 a piece of filter paper, and the whole has a cover to keep the heat 

 in ; as soon as the paraffine is melted it flows through the paper 

 quite rapidly, and comes out perfectly clean. I have learned by 

 experience that the lines, scratches and tears which one gets in 

 sections during paraffine cutting are due usually to the presence of 

 minute particles of dirt in the paraffine, and if the paraffine is filtered 

 and carefully kept free from dirt the edge of the knife will last 

 much longer, and the sections come out much more perfect. The 

 filtering of paraffine may be considered an indispensable process 

 for those who want to make perfect sections. 



The process of imbedding which I ordinarily employ, my work 

 being principally with vertebrate embryos, is as follows: the tissue 

 is kept in 70° alcohol until the process of imbedding begins. For 

 imbedding, the tissue is first dehydrated in 96° alcohol, and then 

 soaked in a mixture consisting of equal parts of oil of cloves and 

 spirits of turpentine for a few hours, varying according to the size 

 of the piece, or sometimes left over night ; it is then transferred to 



