MICROSCOPIC VISION. 161 



more common way of doing it without any stop, and that is 

 by using a narrow con. When a narrow axial cone of i Ru- 

 mination is used, spectra of the first order pass through an 

 intermediate zone of the objective aperture, whilst those of 

 the 2nd order pass through an outer zone (Fig. 2) ; then 

 spherical aberration (which is always present, even in the 

 best objectives, to a far greater extent than is generally 

 supposed) will cause spectra of the 2ud order to be combined 

 with the dioptric beam 7), to the exclusion of those of the 

 1st order, thereby forming a false image. Putting it in a 

 popular form, ic may be said that the eye is quite unable to 

 distinguish whether any given spectra are spectra of the 1st 

 order arising from a fine structure (P, P, Fig. 3) or spectra 

 of the 2nd order from a coarse structure (0, 0, Fig. 2); so 

 that if spectra of the 2nd order and the dioptric beam, Z), 

 are brought into focus together, while spherical aberration 

 is causing the spectra of the 1st order to be out of focus, the 

 result is that the eye interprets the spectra of the 2nd order 

 of the coarse structure (0, 0, Fig. 2) as if they were spectra 

 of the 1st order of fine structure (P, P, Fig. 3), consequently a 

 ghost image of fine structure is seen. If, therefore, the coarse 

 structure (0, 0, Fig. 2), which in this instance would be the 

 true image, had a certain ntimber of lines or marks to the 

 inch, say 12,000, then the ghost image would have precisely 

 double that quantity, or 24,000 to the inch. 



False images of greater complexity may be made by 

 combining spectra of the 3rd order with the dioptric beam 

 Z), when those of the 1st and 2nd orders are excluded, 

 etc. 



All these false images are dispelled by means of the wide- 

 angled axial cone of illumination {i.e. f cone), because it 

 causes groups of spectra of the 1st order to pass through the 

 same zone of the objective as those of the 2nd order, and 



