66 



Modern Microscopy. 



There is yet another form called the Kamsden screw 

 micrometer. This is an expensive piece of apparatus, and 

 consists of an eyepiece containing two wires, one fixed, the 

 other travelling by means of a screw having 100 threads to 

 the inch. The milled head of this screw is divided into 

 100 parts. Across the field are very small equi-distant V- 



FIG. 24. Jackson Micrometer 

 Eyepiece. 



FIG. 25. Kamsden Screw 

 Micrometer. 



shaped teeth, each of which is equal to one complete revo- 

 lution of the milled head. The value of these teeth is taken 

 against the stage micrometer, and the object placed on the 

 stage. The object is then brought against the fixed wire, 

 and the travelling wire moved to the other part that is 

 desired to be gauged. 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 expensive, and with care one 

 of the previous methods named will be, as a rule, sufficient. 



Troughs, Live-cages, Stage Forceps, etc. 



Troughs. These are made of various materials, including 

 glass, vulcanite, brass, etc., and are used for examining 

 infusoria and animalculse alive under the microscope. The 

 essentials of a trough are that a medium power, say 

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



