PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 



177 



IFiG. 119.- 



Ross's Lever of Contact. 



the Tool-shops, answers this purpose very well; but Ross's Lever of Con- 

 tact (Fig. 119), devised for this express purpose, is in many respects pre- 

 ferable. This 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 represents 

 l-1000th 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 deli- 

 c-ate 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 ver- 

 tical plate of 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 inches given by the number on the graduated arc to which the ex- 

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

 the glass is .008 or l-125th of an inch. 1 It will be found convenient to 

 sort the covers according to their thicknesses, and to keep the sortings 

 apart, so that each may be used for the powers to which it is the most 

 suitable. For Objectives whose angle of aperture is between 40 and 75, 

 glass of .008 is not too thick; for Objectives of between 75 and 120 of 

 aperture, the thickness may range from .006 to .004; but for Objectives 

 whose angle of aperture exceeds 120, and whose focus is less than l-10th 

 of an inch, only covers of from .004 to .002 should be used. 



167. On account of the extreme brittleness of the Thin-glass, it is 

 desirable to keep the covers, when cut and sorted, in some fine and soft 

 powder, such as Starch. Before using a cover, however, the Microscopist 

 should be careful to clean it thoroughly; not merely for the sake of 

 removing foulness which would interfere with the view of the object, but 

 also for the sake of getting rid of adherent starch -grains, the presence of 

 which might lead to wrong conclusions; and also to free the surface from 

 that slight greasiness, which, by preventing it from being readily wetted 

 by water, frequently occasions great inconvenience in the mounting of 

 objects in fluid. The thicker pieces may be washed and wiped without 

 much danger of fracture, if due care be employed; but the thinner require 

 much precaution; and in cleansing these, a simple instrument devised by 

 Mr. W. W. Jones will be found very useful. This consists of a small 

 tube of brass about an inch in diameter and the same in height (a stout 

 pill-box makes a good substitute), into which fits loosely a weighted-plug, 

 to the flat bottom of which is cemented a piece of chamois leather. 

 Another piece of soft leather is stretched upon a flat tablet of wood or 

 plate-glass; and by placing the cover-glass (damped by the breath) under 

 the plug; within the end of the tube, and keeping the tube well-down on 

 the tablet, the glass can be rubbed between the two leather surfaces with 

 perfect security, the weight of the plug affording sufficient pressure. 1 



1 Another form of gauge, in which the measurement is obtained with great 

 precision and facility by the sliding of a wedge, is described in the " Journ. of 

 the Roy. Microsc. Soc.," Vol. ii. (1879), p. 65. , 



1 In the improved form of this little instrument made by Messrs. Hunter & 

 12 



