228 



USE OF THE MICKOSCOl'E. 



those on the rule, will be seen at one and the same time; 

 and, after a little practice, it will be found a very easy matter 

 to count how many divisions of the rule are covered by two 

 or more of the micrometer. Thus, for example, suppose that 

 each division of -j^-g of an inch occupies just one inch of 

 the rule, the magnifying power will then be one hundred 

 times, as j^^ of an inch is made as large as one inch ; if the 

 same divisions, either with another object-glass or eye-piece, 

 correspond to one-and-a-half inch of the rule, then the power 

 will be one hundred and fifty diameters. The magnifying 

 powers of the different object-glasses, with each of the eye- 

 pieces, should, also, for convenience of reference, be set down 

 in a tabular form. Mr. Ross supplies with his microscopes 

 a table of the following construction, having appended to 

 it, that described at page 221, with the value of the divisions 

 of the eye-piece micrometer with each of the object-glasses. 

 For the use of those who have draw-tubes to their micro- 

 scopes, another column might be added to denote the distances 

 it must be drawn out, in order that the value of the micrometer 

 divisions and the magnifying power may be set down in even 

 numbers. 



Several other methods, besides those now described, have 

 been from time to time adopted, to ascertain the magnifying 

 power of the compound microscope; but as none are so accurate 

 as that of Hooke, it has been thought vumecessary to describe 



