ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 
365 
The law, then, for the manufacture of the simplest form of false 
image, is the union of a spectrum of the second order with D, when that 
of the first order is suppressed. In this case the false image will consist 
of a doubling of line structures, and the insertion of an intercostal in 
hexagonal and similar structures. To repeat, the cause of the false 
image is the suppression of (1) and not merely the admission, per se, of 
Fig. 48. 
(2); for if (2) is admitted and combined with the dioptric beam, (1) 
being also combined with them, there will be no false image. This effect 
can be obtained by a suitable stop, or by using a narrow cone. When 
a narrow axial cone is used, spectra of the first order pass through an 
intermediate zone of the objective aperture, whilst those of the second 
order pass through an outer zone (fig. 48) ; then spherical aberration 
Fig. 49. Fig. 50. 
(which is always present, even in the best objectives, to a far greater 
extent than is generally supposed) will cause spectra of the second 
order to be combined with D, to the exclusion of (1), thereby forming a 
false image. Putting it in a popular form, it may be said that the eye 
is quite unable to distinguish whether any given spectra are spectra of 
