CAMPANl'S EYE-PIECE: COMBINED ACTION WITH OBJECTIVE. 4L 



Iii Fig. 9 let a b be the source of light, the extent of which is 

 limited by m m. Let the angle at which the rays issuing from the 

 points a and I intersect in the focus of the Microscope be 60, and 

 the angular aperture of the objective at least as large. If we draw 

 a line i r parallel to the axis from the point i, where the ray a g, 

 which passes through the focus, cuts the anterior principal plane, it 

 represents the path of the incident ray. A second ray, drawn from 

 a to the anterior principal point, passes at a similar angle through 

 the posterior principal point, and cuts the other ray in the point a, 

 which is therefore to be regarded as the image-point. Similarly, 

 we obtain ft ; a /3 is therefore the image of the source of light. It 

 will, of course, lie the nearer to the focal plane F* the greater the 

 distance of a b ; its diameter, however, will remain unaltered as 



' F* 



FIG. 9. 



long as the angle of the incident marginal rays = 60, as we have 

 assumed. It is, therefore, only necessary to determine this con- 

 stant diameter for the simplest case, which is when the distance of 

 the source of light is infinite, and when, consequently, its image 

 lies in the focal plane. Then half the diameter of the image is 

 evidently given by the tangent of half the angle of aperture 

 multiplied by the distance E* F*, that is, by the focal length of 

 the Microscope. We get, therefore, for the whole diameter, 



a Q = 2 x tan 3 - X focal length = 2 X '577 x '611 = '7 mm.; 

 f 



