39 



overrun with cheap foreign productions, the most 

 fanciful claims were made in this direction. Avoid 

 strolling or street venders who, as a rule, not only 

 make the most ridiculous claims as to magnifying 

 power, but charge much higher prices than the 

 same articles can be bought for from reliable 

 opticians and generally offer articles of worthless 

 or at best doubtful value. 



Some precaution should also be used in refer- 

 ence to quality in purchasing a magnifier. As 

 competition causes a downward tendency in prices, 

 it unfortunately often involves a deterioration in 

 quality. The ordinary forms are mounted in 

 vulcanite ; black horn is often palmed off as such, 

 but is a poor substitute as it warps and cracks. 

 The surfaces of lenses, instead of being perfectly 

 polished are often scratched and unfinished show- 

 ing small pit-holes or undulating surfaces. This 

 is common among cheap Coddington lenses and 

 naturally prevents obtaining a distinct image. 



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 four times in the linear, it is sixteen 

 times in area. The commonly accepted term to 

 express magnifying power of simple, 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 five diameters ; 



OP THE 



TJNIVERSmr 



