342 Dr. Gr. J. Stoney on Microscopic Vision. 



it diminishes the inclination of these beams to one another. 

 This brings about two desired results : it enlarges the image, 

 and it makes it possible for its constituent beams, after they 

 have passed the focal image, to be collected by the eyepiece 

 and transmitted through so small an opening as the pupil of 

 the eye, instead of diverging over the great extent to which 

 they were spreading when they left the object. 



13. Useless work done by the objective. — But the objective 

 cannot accomplish this useful work without at the same time 

 producing other effects which are undesirable. Thus, it 

 transforms the beams of plane waves into convex beams, as 

 explained in § 10. This somewhat distorts the image. The 

 image is still more distorted in the direction of the line of 

 sight, whereby any elevation on the object is shown as unduly 

 prominent in the image *. Neither of these distortions, 

 however, would cause the amount of detail in the microscopic 

 image to fall short of that in the standard image. 



That which above all produces this defect, and produces it 

 however well the spherical and chromatic aberrations of the 

 objective may have been corrected, is that the angular aper- 

 ture of the objective falls short of 180°. With the best 

 immersion-lenses the angular aperture is about 120° or 130°, 

 so that little more than half the light would be caught by 

 the objective, if the light were emitted equally in all direc- 

 tions. One part of the light thus excluded is that which 

 in the standard image brings out the finest part of the detail 

 which that image can reach, since it is the light which pro- 

 duces the finest of the rulings that form the standard image. 



There is another imperfection consequent upon this exclu- 

 sion of part of the light emitted by the object, viz., the 

 intrusion into the microscopic image of intercostal markings, 

 false resolutions, a general haze of light, and so on — additions 

 to the image and other alterations of it which have nothing 

 to correspond to them either in the object under examination 

 or in its standard image. The following is perhaps the easiest 

 way of understanding how they arise. 



14. The Visual Substitute. — In order to study microscopic 

 vision, or vision of any kind, with full effect, it is well to 

 begin with the consideration that what we seem to see with 

 the naked eye is never the natural object itself, nor is it an 

 enlargement of it when we examine it through a microscope 

 or telescope. What we see is, in fact, only a visual substitute 

 for the real object in the first case, and for an enlargement of 

 the same when we use an instrument ; and the study of 



* This distortion may be traced by an elementary investigation in 

 geometrical optics. 



