82 NATURALISTS’ ASSISTANT. 
less amount of amplification. This is frequently obtained by 
an arrangement similar to that shown in fig. 30. For very 
low powers it is convenient to have the style of mounting 
used by watchmakers as this can readily be held by the 
muscles around the eye, leaving both hands free for work. 
With the simple lens there are, however, disadvantages ; as, 
when a clear view is obtained of the centre, objects at the 
margin of the field are blurred and surrounded by rainbow 
hues. 
Various plans have been adopted to avoid these defects 
a 
Fic. 31. Fic. 32. 
(known as spherical and chromatic aberration), the simplest 
is that of the Coddington lens (fig. 31) in which the glass 
has a deep groove cut around the margin which is filled with 
some black pigment leaving only a small opening in the 
centre. This to a considerable extent does away with the 
color and distortion but at the expense of the brilliancy of 
the view obtained. 
Another, and the better, method of avoiding aberration is 
by having the lens made of different kinds of glass, which 
produce different effects on light and which tend to balance 
each other. These lenses are sold under the names of 
