MARVKLS OF POXD-LIFE. 



having at least two powers, one with a focus of an 

 inch or two-thirds of an inch, and the other of 

 half or a quarter. Cheap microscopes have usually 

 only one eye-piece, those of a better class have 

 two, and the best are furnished with three, or 

 even more. 



The magnifying power of a compound microscope 

 depends upon the focal length of the object-glass, 

 (or glass nearest the object,) upon tbe length of 

 the tube, and the power of the eye-piece. Witli 

 regard to object-ghisses, those of shortest focjil 

 length have the higliest powers, and the longest 

 eye-pieces liave the lowest powers. The body of a 

 microscope, or principal tube of which it is com- 

 posed, is, in the best instruments, about nine inches 

 long, and is furnished with a draw tube, capable of 

 being extended six inches more. From simple 

 optical principles, the longer the tube the higher 

 the power obtained with the same object-glass; but 

 only object-glasses of very perfect consti'uction will 

 bear the enlargement of their own imperfections, 

 which results from the use of long tubes; and con- 

 sequently for cheap instruments the opticians often 

 limit the length of the tube, to suit the capacity 

 of the object-glasses they can afford to give for 

 trhe money. Such microscopes may be good enough 

 fm' the generality of purposes, but they do not, 

 with glasses of given focal length, afford the same 



