11 



jecting, say a flame or window frame, upon a white 

 piece of paper ; the distance between the paper to the 

 center of the lens, when the image is most distinct, is 

 its focal distance. When a lens is two inches or more 

 in diameter, it is usually .termed a reading glass. 



Using Magnifiers. In using magnifiers the lens 

 should be held close to the eye and such a position 

 taken that the object will receive the best illumina- 

 tion. In the lenses of equally convex surfaces, it is 

 immaterial which side is held toward the eye ; but 

 when plano-convex lenses are used, the plane side 

 should always be toward the eye, as it gives the flat- 

 test field. 



Aberrations. Two factors arise which prevent 

 the advantageous use of more than about 25 diame- 

 ters in magnifiers ; they are called the chromatic and 

 spherical aberrations. The first is the term employed 

 when the object is apparently fringed with color, pre- 

 dominently blue and yellow ; the second, when all but 

 the central portion of the lens shows the object indis- 

 tinctly ; these 

 faults increase 

 with the magni- 

 fying power. In 

 the case of a 

 combination of 

 Fig- 2 - several lenses, Fi s- 3- 



they may partially be overcome by interposing an 

 opaque plate with a small opening, called a diaphragm, 

 between them, which cuts off the outer or marginal 



