THE CODDINGTON LENS. 



39 



larger field of view, and is equally good in all 

 directions, as it is evident that the pencils a b and & a 

 pass through under precisely the same circumstances. 

 Its spherical form has the further advantage of render- 

 ing the position in which it is held of comparatively 

 little consequence. It is very generally used as a hand 

 magnifier ; but its definition is certainly not so good as 

 that of a well-made doublet or achromatic lens. It is 

 usually set in a folding case, as represented in the 

 figure, and so contrived as to be admirably adapted 

 for the waistcoat-pocket. It is sold with the small 

 holder, fig. 23#, for holding and securing small. objects 



Fio. 23. 



FIG. 23a. 



during examination. Browning's Platyscopic Pocket 

 Lens is a useful form for botanists and mineralogists. 

 Its focus is about three times farther from the object 

 than the Coddington, and allows of opaque objects 

 being easily examined ; it has also three degrees of 

 magnifying power of 15, 20, and 30 diameters^ 



When the magnifying power of a lens is consider, 

 able, or when its focal length is short, and its proper 

 distance from the object equally short, it then becomes 

 necessary to be placed at a proper distance with great 

 precision; it cannot therefore be held with sufficient 



