THE MICKOSCOPE. 



Fig. 12. 



this provision with a proper assortment of object-pieces and eye- 

 pieces, all possible gradations of power between the highest 

 attainable, and the lowest which is applicable, can be obtained. 



The actual magnitude of the space which can be presented at 

 once to the view of the observer, will vary with the magnifying 

 power ; but in all cases it is extremely minute. Thus, with the 

 lowest class of powers, where it is largest, it is a circular space, 

 the diameter of which does not exceed the 8th or 10th of an inch ; 

 it follows, therefore, that no object, the extreme limits of whose 

 linear magnitude exceed this, can be presented at once to the 

 view of the observer. Such objects can only be seen in their 

 ensemble, by means of less powerful magnifying glasses, or with 

 the naked eye. 



22. The field of view, with powers from 100 to 300, varies in 

 diameter from the loth to the 40th of an inch ; from 300 to 500 

 it varies from the 40th to the 70th of an inch ; and from 500 to 

 700 from the 70th to the 100th of an inch. 



It will thus be understood, that even with the moderate power 

 of 700, an object to be included wholly within the field of view, 

 must have a magnitude such as may be 

 included within a circle whose diameter 

 does not exceed the 100th of an inch. 

 These observations will be more clearly 

 appreciated by reference to the annexed 

 diagram, fig. 12, where A is a circle 

 whose diameter is the 6th of an inch ; 

 B one whose diameter is the 12th of an 

 inch ;"c the 25th, D the 50th, and E the 

 100th? 



But when still higher powers are used, 

 the actual dimensions of the entire space 

 comprised within the field of view will be 

 so very minute, that an object which 

 would fill it, and still more, smaller 

 objects included within it, would not only 

 be altogether invisible to the naked eye, 

 but would require considerable micro- 

 scopic power to enable the observer to see them at all. 



The actual dimensions of the field of view, which correspond to 

 each magnifying power, vary more or less in different instru- 

 ments. Those which I have given above, are taken from a 

 microscope made by Charles Chevalier, which is in my possession. 

 The difference however in this respect, between one instrument 

 and another, is not considerable, and the above will serve as a fair 

 illustration of the limits of the field of instruments in general. 

 26 



