246 Modern Microscopy 



The micrometer screw is fitted with a large wheel having 

 its periphery divided ; it is also supplied with a vernier. 

 The whole arrangement in its simplest form may be seen 

 in Pig. 57, in which the wheel is rotated by hand to the 

 required number of peripheral divisions. This is a tedious 

 operation, but by the aid of an ingenious mechanism the 

 focusing can be done without looking at the wheel, by 

 simply moving a lever to and fro with the finger. This 

 focusing apparatus is seen in Fig. 65. The pitch of the 

 micrometer screw is twenty threads to the centimetre ; con- 

 sequently one revolution equals 0"5 millimetre. The wheel 

 has 100 large teeth cut into its periphery, therefore an 

 advance of 1 tooth equals 0'005 millimetre, or 5 fi. The 

 wheel is about 110 millimetres in diameter. 



The focusing apparatus consists of a sector, the arm to 

 which the arc is fixed being secured at the centre of curva- 

 ture to a longer arm upon which the sector rotates to the 

 extent of its arc. The long arm is attached to, and rotates 

 about, the micrometer screw, its free end forming the guide 

 through which the periphery of the sector travels. The 

 arc has engraved upon its upper surface a series of figures 

 according with a certain number of teeth of the large- 

 toothed wheel. The figures are 0-005, O'OIO, 0-015, 0-020, 

 0-025, 0-030, 0-035, 0-040, 0-045, 0-050, being decimal 

 fractions of a millimetre, corresponding from 1 tooth of 

 the wheel to 10 teeth. As 1 tooth of the wheel equals 

 5 /i, the values upon the arc will be from 5 /u to 50 fi. The 

 sector is set to any required number upon the arc, and fixed 

 by means of the hexagonal rod seen projecting at the base 

 of Pig. 65. This rod is terminated by a screw which passes 

 through the end of the long arm into any of the ten holes 

 at the edge of the arc, thus fixing the arm at the required 

 position. A pawl is placed upon the long arm, which upon 

 the forward movement engages a tooth of the wheel, but 

 slips over the teeth in the backward movement. The move- 

 ment of the sector is limited by the two uprights projecting 

 from the base of the instrument. After an advance of the 



