236 



Modern Microscopy 



with a vernier in the form of a divided drum, the two halves 

 being symmetrical, and the screw passing through the axis 

 of the drum. Each half of the drum has cut into its peri- 

 phery a set of notches, some of which are numbered as 

 follows : 1, 2, 3, 15 ; these are the vernier numbers. The 

 vernier is set by means of a steel pointer fitting into holes in 

 the periphery of the drum. The vernier numbers, 1, 2, 8, 15, 

 correspond to the i, I, ^, -f-g of a revolution of the drum. 

 To set the drum to any of these portions of its revolution, 

 the numbers upon the two halves of the drum corre- 

 sponding to the portion of revolution required must be 

 made to coincide by means of the steel pointer previously 

 mentioned. 



Suppose, for example, we bring the numbers 3 and 3 

 to so coincide that the notches upon each half of the drum 

 opposite 3 and 3 are continuous. Then a spring may be 

 set by means of a lever adjustment ; when set, the knife 

 edge of the spring presses upon the periphery of the drum, 

 and as it rotates the knife-edge fits with a loud click into 

 the notches that are in apposition. Thus, having set the 

 numbers 3 and 3 to coincide, three sharp clicks will be 

 heard for each revolution of the drum, and at each click 

 the object will have been raised 0'005 millimetre, or 5 f«. 

 The following table may be of use to the beginner : 



At first sight it would appear that only four thicknesses 

 of sections could be cut by this method, but so cunningly 

 are these numbers arranged that, by giving the requisite 

 number of clicks successively, almost any thickness of 

 section may be arrived at. Thus, for sections 10 f^ thick. 



