628 



Transactions of the Society. 



Other Methods of Measuring. — (1) The Ramsden Eye-piece 

 Micrometer. — This is a convenient and sensitive method, but e very- 

 measurement taken by it has to be divided by the magnifying 

 power of the objective, a quantity which is only known to a 

 certain degree of accuracy. Hence, besides the error due to the 

 micrometer reading, a second one is introduced into the measure- 

 ment. Moreover, the magnifying power must be known for every 

 objective. Again, in most cases the Eamsden eye-piece is mounted 

 in the tube of the Microscope, so that movement and oscillations 

 of the tube take place each time the Microscope is handled, thus 

 introducing errors. To avoid this, the eye-piece should be on a 

 separate stand. 



(2) Less sensitive than the former is the Jackson eye-piece 



Fig. 127. 



micrometer. It is convenient, as with it the eye-piece is not 

 handled, and the difficulty in the Eamsden form is avoided. But 

 it labours under the other disadvantages mentioned. 



(3) The stage micrometer method. — Whilst the ruled scale is 

 the standard of length it cannot be used directly on the object to 

 be measured, but either we (a) use the camera lucida method of 

 sketching, which is clumsy, or (b) calibrate the eye-piece micro- 

 meter by means of the ruled surface, and then proceed hence- 

 forward to use the eye-piece micrometer. The stage micrometer 

 rulings are not perfectly uniform.* Thus the methods in common 

 use at present are all indirect, for at least two steps have to be 



* See Carpenter's book on the Microscope, 7th edition, p. 231. 



