Accessory Apparatus 117 



taken against the stage micrometer, and the object placed 

 on the stage. One edge of the object is then brought 

 against the fixed wire, and the travelling wire moved to the 

 other part that it is desired to gauge. By then counting 

 the number of intervening teeth, and reading the fraction 

 on the milled head, it can at once be ascertained what 

 magnifying power is used. This is considered the most 

 accurate and precise method of working, but it is an ex- 

 pensive piece of apparatus, and with care one of the previous 

 methods named will be, as a rule, sufficient. 



Persons having abnormal vision are likely to make errors 

 in measuring. To obviate this, a cap carrying a lens that 



FIG. 45. JACKSON MICRO- FIG. 46. RAMSDEN SCREW 



METER FITTED TO EYEPIECE. MICROMETER. 



will correct the abnormality should be placed over the top 

 of the eyepiece, as described on page 78, while measure- 

 ments are being taken. 



Troughs, Live-cages, Stage Forceps, etc. 



Troughs. These are made of various materials, including 

 glass, vulcanite, brass, etc., and are used in the examination 

 of infusoria and animalculse alive under the microscope. 

 The essentials of a trough are that a medium power, say 

 inch at least, can be used, that it may be easily cleaned, 

 and that if broken it can be repaired. The ordinary 



