44O PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS 



kinds ; and hence it should only be employed for the purpose for 

 which it is absolutely needed. The thickest pieces, again, may be 

 most advantageously employed as covers for large cells, in which 

 objects are mounted in fluid to be viewed by the low powers whose 

 performance is not sensibly affected by the aberration thus produced. 

 The working microscopist will find it desirable to provide himself 

 with some means of measuring the thickness of his cover-glass ; and 

 this is especially needed if he is in the habit of employing objectives 

 without adjustment, which are corrected to a particular standard. 

 A small screw-gauge of steel, made for measuring the thickness of 

 rolled plates of brass, and sold at the tool-shops, answers this purpose 

 very well ; but Ross's lever of contact (fig. 372), devised for this 

 express purpose, is in many respects preferable. This consists of a 

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

 at one end an arc graduated into twenty divisions, each of which re- 

 presents the r oVo^h of an inch, so that the entire arc measures the 

 5-fi th 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 it against a vertical plate of steel that is screwed to the 



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FIG. 372. Ross's lever of contact. 



horizontal table. The piece of thin glass to be measured being in- 

 serted 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 the -^th 

 of an inch. It will be found convenient to sort the covers according 

 to their thicknesses, and to keep the sortings apart, so that there 

 may be a suitable thickness of cover for each object. But it is well 

 to remember that, with the exception of objects to which from their 

 size or nature it is impossible to apply high powers, it is better to 

 mount the object so that, if it be required or desirable, high powers 

 may be used upon it. 



Another simple and very efficient cover-glass tester is made by 

 Zeiss, of Jena, and illustrated in fig. 373. It will be seen that the 

 measurement is effected by a clip projecting from a box, between the 

 jaws of which the cover to be measured is placed ; the reading is 

 given by an indicator moving over a divided circle on the upper face 

 of the box. The divisions show hundredths of a millimetre, and the 

 instrument measures to upwards of 5 mm. 



