sists mainly in the large variety of glass with different 

 ratios of refractive index and dispersive power, and its 

 effectiveness in lens making depends entirely on its 

 intelligent use by the optician. 



As defects in the objective have already been 

 specially mentioned, it will at this point be well to state 

 that while an achromatic lens is unnecessary in the 

 eyepiece, a supplementary lens below and near the 

 upper lens has been found beneficial in so affecting the 

 image, by collecting the rays, that it can be viewed at 

 one glance and without spherical or chromatic aberra- 

 tion. For this reason the lower lens is called the 

 field or collective lens, and the upper lens the eye lens. 

 These lenses are mounted in a tube with fixed 

 relations, and are then called the eyepiece or ocular. 

 In the diagram, Fig. 32, the course of rays from the 

 object through the objective and eyepiece is shown. 

 g h represents the objective, / / the field lens and op 

 the eye lens of the eyepiece. As the rays from the 

 object pass through the objective they are seen to cross 

 before reaching the field lens, are converged as they 

 pass through, and further converged by the eye lens op. 

 At the point c d they form a real image of the object, 

 which can readily be seen by placing a ground glass or 

 piece of oiled paper at this point. It is an interesting 

 experiment and one which we recommend trying. 

 The eye lens enlarges this image and forms a greatly 

 magnified virtual image at e f. From this diagram 



58 



