PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 217 



the security of the mounting. It is to secure this, that the Author pre- 

 fers rings of tin ( 171) to those of glass, for cells of moderate depth; for 

 their surface can be easily made perfectly flat by grinding with water, 

 first on a piece of grit, and then on a Water- of- Ayr stone these stones 

 having been previously reduced to a plane surface ( 193). If glass rings 

 are not found to be ' true ' they must be ground-down with fine emery 

 on a plate of lead. When the cell has been thus finished-oil, it must be 

 carefully cleaned-out by syringing into it some of the mounting-fluid; 

 and should be then examined under the Dissecting Microscope for minute 

 air-bubbles, which often cling to the bottom or sides. These having 

 been got rid of by the needle, the cell should be finally filled with the 

 preservative liquid, and the object immersed in it, care being taken that 

 no air-bubbles are carried-down beneath it. The cell being completely 

 filled so that the liquid is running over its side, the cover may then be 

 lowered down upon it as in the preceding case; or, if the cell be quad- 

 rangular, the cover may be sloped so as to rest one margin on its wall and 

 fresh liquids may be thrown in by the Syringe, while the other edge is 

 lowered. When the cover is in place, and the liquid expelled from it 

 has been taken up by the syringe, it should again be examined under a, 

 lens for air-bubbles; and if any of these troublesome intruders should 

 present themselves beneath the cover, the slide should be inclined, so as. 

 to cause them to rise towards the highest part of its circumference, and 

 the cover slipped away from that part, so as to admit of the introduction, 

 of a little additional fluid by the pipette or syringe; and when this 

 has taken the place of the air-bubbles, the cover may be slipped back into 

 its place. The surface of the ring and the edge of the cover must then 

 be thoroughly dried with blotting-paper, care being taken that the fluid be 

 not drawn away from between the cover and the edge of the cell on which 

 it rests. These minutiae having been attended to, the closure of the cell 

 may be at once effected by carrying a thin layer of Gold-size or Dammar 

 around and upon the edge of the glass-cover, taking care that it touches 

 every point of it, and fills the angular channel which is left along its 

 margin. The Author has found it advantageous, however, to delay 

 closing the cell for some little time after the superfluous fluid has been 

 drawn off; for as soon as evaporation from beneath the edge of the cover 

 begins to diminish the quantity of fluid in the cell, air-bubbles often begin 

 to make their appearance, which were previously hidden in the recesses 

 of the object; and in the course of half an hour, a considerable number 

 are often collected. The cover should then be slipped aside, fresh fluid 

 introduced, the air-bubbles removed, and the cover put on again; and this, 

 operation should be repeated until it fails to draw forth any more air- 

 bubbles. It will of course be observed, that if the evaporation of fluid 

 should proceed far, air-bubbles will enter beneath the cover; but these 

 will show themselves on the surface of the fluid; whereas those which 

 arise from the object itself are found in the deeper parts of the cell. 

 When all these have been successfully disposed of, the cell maybe ' sealed ' 

 and ' ringed ' in the manner already described. 



213. Importance of Cleanliness. The success of the result of any of 

 the foregoing operations is greatly detracted-from, if, in consequence of 

 the adhesion of foreign substances to the glasses whereon the objects are 

 mounted, or to the implements used in the manipulations, any extraneous 

 particles are brought into view with the object itself. Some such will 

 occasionally present themselves, even under careful management; espe- 

 cially fibres of silk, wool, cotton, or linen, from the handkerchiefs, etc.,. 



