10 



mon are those with one or several double-convex 

 lenses (Fig.l). The shorter the radii (the more curved 

 the surfaces) are in these, the greater will be the mag- 

 nifying power, and the higher this is, the less of the 

 object's surface can be seen at once. Each addi- 

 tional lens increases the magnifying power in pro- 

 portion to its curvature. The distance between the 

 lens and the object, when this is seen most distinctly, 

 is called the focus ; at the point where the object is 

 most distinct, the lens is said to be in focus ; when 

 indistinct or blurred, out of focus. 



Magnifying Power. Unless a microscope is 

 known to come from the hands of a reliable firm, any 

 claim as to magnifying power should be accepted with 

 reserve. In former years, when the country was over- 

 run with cheap foreign productions, the most fanciful 

 claims were made in this direction. It is evident that 

 a lens magnifies an object equally in all directions ; 

 this is said to be in areas, and is the square of the 

 linear, so that if an object is magnified 4 times in 

 the linear, it is 16 times in area. The commonly 

 accepted term to express magnifying power of sim- 

 ple, as well as compound microscopes, is in diameters 

 (linear}. A single lens of 1 inch focus magnifies 

 about ten diameters ; one of 2 inch focus, about 5 

 diameters ; one of \ inch focus, 20 diameters, and so 

 on. In a lens of high magnifying power, the focus is 

 ordinarily about twice the diameter, so that if a lens 

 is \ inch diameter its focus is about 1 inch. This 

 may, however, be more accurately determined by pro- 



