216 



MOUNTING OF OBJECTS. 



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

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

 the microscope (holding it in the stage-forceps), and measuring 

 its edge by the eye-piece micrometer (§ 46). A much more ready 

 means is aiforded, however, by the Lever of Contact (Fig. 60) de- 

 vised by Mr. Ross for this express purpose. This instrument 



Fig. 60. 



Lever of Contact. 



consists of a small horizontal table of brass, mounted upon a 

 stand, and having at one end an ai'c graduated into 20 divisions, 

 each of which represents 1-lOOOth 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 ot 

 steel that is screwed to the horizontal table. The piece of thin 

 glass to be measured, being inserted between the vertical plate 

 and the projecting 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 l-125th of an inch. A very 

 elegant little instrument, which is used by watchmakers for mea- 

 suring the thickness of thin plates and wires, may be obtained 

 at a much less cost from the dealers in Swiss tools ; this answers 

 the purpose equally well; but the "value" of its scale must be 

 determined by gauging the thickness of a piece of glass, or the 

 diameter of a fine wire, and comparing the number of divisions 

 which it indicates, with the micrometrical measurement of the 

 same body obtained by the microscope. When the glass covers 

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

 convenient to employ those of one particular thickness for each 

 particular class of objects ; since, when one object is being ex- 

 amined after another, no readjustment of the objective will then 

 be required for each. This will be found a great saving of time 

 and trouble, when high powers are in use. It is undesirable to 

 employ glass covers of greater thickness than l-140th (-007) of 

 an inch, with any object-glass whose aperture exceeds 75° ; and 

 for object-glasses of 120° and upwards, the glass cover should not 

 exceed l-250th (-004) of an inch. 



119. On account of the exti'eme brittleuess of the thin glass, 

 it is desirable to keep the pieces, Avhen cut and sorted, in some 



