820 Transactions of the Society. 



Microscope is a fallacious one, as it supposes a constant tube- 

 length ; whilst, as we have seen, the true tube-length varies with 

 the different objectives and eye-pieces used. 



To determine the power of the Microscope from the powers of 

 the eye-piece and objective, it is necessary, in addition, to know 

 the position of the focal planes of each of the latter. How these 

 may be readily determined must be deferred for a subsequent 

 occasion. 



