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 wcrk. 



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 



FIG. 31. 



FIG. 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 



