162 



TESTING THE MICROSCOPE. 



the traditional method of description we will endeavour to explain 



the action of the Cam- 

 pani eye-piece by a 

 construction, in which 

 all essential factors 

 are taken into account. 

 Let A (Fig. 95) be the 

 objective, B the field- 

 lens, and the eye- 

 lens. We may take 

 for the object the 

 whole field of view, or 

 a small aperture in an 

 opaque plate, &c. ; the 

 condition necessary is 

 that the emergent 

 cones of rays occupy 

 the whole aperture of 

 the objective, or its 

 central part. On this 

 supposition, the differ- 

 ently coloured rays are 

 evidently so refracted 

 that the emergent 

 cones corresponding to 

 a given object-point 

 have a common axis. 

 The lines of direction 

 in which the apices 

 of differently coloured 

 cones lie, are therefore 

 determined ; but the 

 plane of these apices 

 is unknown, and is 

 dependent on the chro- 

 matic aberration of 

 the objective. In our 

 figure, the two syni- 

 FlG - 95 ' metrical cones of light 



to the right and left of the middle line correspond to two opposite 



