THIN-GLASS COVERS. LEVER OF CONTACT. 



203 



which are to be viewed by the highest powers. The thickest pieces, 

 again, may be most advantageously employed as covers for large 

 Cells in which objects are mounted in fluid (§§ 168-170), to be 

 viewed by the low powers whose performance is not sensibly affected 

 by the aberration thus produced. And the pieces of medium thin- 

 ness will be found most serviceable for all ordinary purposes ; 

 neither being, on the one hand, difficult to handle, nor, on the 

 other, interfering with the clearness of the image formed by medium 

 powers of moderate aperture, even when no special adjustment is 

 made for the aberration they occasion (§ 131, v.). 



150. The exact thickness of any piece of glass may be deter- 

 mined without difficulty, by placing it edgeways on the stage of the 

 Microscope (holding it in the stage-forceps), and measuring its edge 

 by the Eye-piece Micrometer (§ 68). A much more ready means is 

 afforded, however, by the Lever of Contact (Fig. 96) devised by 

 Mr. Ross for this express purpose. This instrument consists of a 

 small horizontal table of brass, mounted upon a stand, and having 

 at one end an arc graduated into 20 divisions, each of which 



Fig. 96. 



Lever of Contact. 



represents 1-1 000th of an inch, so that the entire arc measures 

 l-50th of an inch ; at the other end is a pivot, on which moves a 

 long and delicate lever of steel, whose extremity points to the 

 graduated arc, whilst it has very near its pivot a sort of projecting 

 tooth, which bears at * against a vertical plate of steel that is 

 screwed to the horizontal table. The piece of Thin Grlass to be 

 measured, being inserted between the vertical plate and the pro- 

 jecting tooth of the lever, its thickness in thousandths of an inch is 

 given by the number on the graduated arc to which the extremity 

 of the lever points. Thus, if the number be 8, the thickness of 

 the glass is *008 or 1-1 25th of an inch. When the glass covers 

 have been sorted according to their thickness, it will be found con- 

 venient to employ those of one particular thickness for each par- 

 ticular class of objects ; since, when one object is being examined 

 after another, no re-adjustment of the Objective will then be required 

 for each. This will be found a great saving of time and trouble, 



