GRINDING AND POLISHING 509 



made over the whole. If this be carefully done, even a very large 

 section may be attached to glass without the intervention of any air 

 bubbles. If, however, they should present themselves, and they 

 cannot lie expelled by increasing the pressure over the part beneath 

 which they are. or by slightly shifting the section from side to side, 

 it is better to take the section entirely oft", to melt a little fresh 

 balsam upon the glass, and then to lay the section upon it as before. 

 When the section has been thus secured to the glass, and the 

 attached part thoroughly saturated (if it be porous) with hard 

 Canada balsam, it may be readily reduced in thickness, either by 

 grinding or filing, as before. or, if the thickness be excessive, by 

 taking oft' the chief part of it at once by the slitting wheel. So 

 soon, however, as it approaches the thinness of a piece of ordinary 

 card, it should be rubbed down with water on one of the smooth 

 stones previously named, the glass slip being held beneath the 

 fingers with its face downwards, and the pressure being applied 

 with such equality that the thickness of the section shall be (as 

 nearly as can be discerned) equal over its entire surface. As soon 

 as it begins to be translucent, it should be placed under the micro- 

 scope (particular regard being had to the method of illumination 

 so as not to flood the object with light), and note taken of any 

 inequality; and then when it is again laid upon the stone, such 

 inequality may be brought down by making special pressure with 

 the forefinger upon the part of the slide above it. When the 

 thinness of the section is such as to cause the water to spread 

 around it between the glass and the stone, an excess of thick- 

 ness on either side may often be detected by noticing the smaller- 

 distance to which the liquid extends. In proportion as the sub- 

 stance attached to the glass is ground away, the superfluous 

 balsam which may have exuded around it will be brought into con- 

 tact with the stone ; and this should be removed with a knife, 

 care being taken, however, that a margin be still left round the edge 

 of the section. As the section approaches the degree of thinne^ 

 which is most suitable for the displav of its organisation, great care 

 must be taken that the grinding process be not carried too far ; and 

 frequent recourse should be had to the microscope, which it is 

 convenient to have always at hand when work of this kind is being- 

 carried on. There are many substances whose intimate structure 

 can only be displayed in its highest perfection when a verv little 

 more reduction would destroy the section altogether: and every 

 microscopist who has occupied himself in making such preparations 

 can tell of the number which he has sacrificed in order to attain 

 this perfection. Hence, if the amount of material he limited, it is 

 advisable to stop short as soon as a i/oml section has been made, and 

 to lay it aside ' letting well alone ' whilst the attempt is heini: 

 made to procure a better one: if this should fail, another attempt 

 may be made, and so on. until either success has been attained or 

 the whole of the material ha.> been consumed : the first section, 

 however, still remaining, whereas, if the tirsi. like every subsequent 

 section, be sacrificed in the attempt to obtain perfection, no tract 

 will be left 'to show what once has been." In judging of the 



