366 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the first order arising from a fine structure (P, P, fig. 49), or spectra of 
the second order from a coarse structure (0, 0, fig. 48) ; so that, if (2) 
and D are brought into focus together while spherical aberration is 
causing (1) to be out of focus, the result is that the eye interprets (2) 
of the coarse structure (0, 0, fig. 48) as if they were (1) of fine structure 
(P, P, fig. 49), consequently a ghost image of fine structure is seen. If 
therefore the coarse structure (0, 0, fig. 48) which in this instance would 
be the true image, had a certain number of lines or marks to the inch, 
say 12,000, then the ghost image would have precisely double that quan- 
tity, or 24,000 to the inch. 
False images of greater complexity may be made by combining (8) 
with D, when (1) and (2) are excluded, &c. 
All these false images are dispelled by means of the wide angled 
axial cone of illumination (i.e. 3/4 cone), because it causes groups of (1) 
to pass through the same zone of the objective as those of (2), and unites 
as well portions of (1) with D (fig. 50), thus rendering their separation 
impossible. 
Fig. 51. 
Fig. 52. 
/ 
In fig. 50, D, (1), and (2) are distinguished by being drawn in steps, 
the overlapping portions being indicated by thick lines. The structure 
O, O, is supposed to have 12,000 lines per inch, and to be similar to that 
in fig. 48. P, P, in figs. 49 and 52, are supposed to have 24,000 lines per 
inch, while S, S, in fig. 51, have 48,000. In these figures the refraction 
of the lens has been omitted. 
“ Oblique illumination ” is another form of the small cone, and was 
probably invented by Dr. Goring in 1826. The dioptric beam passes 
through a marginal zone on one side ; and when S, S, is barely resolved 
(1) passes through the same zone on the opposite side (fig. 51) ; duplica- 
tion is then impossible, and the image will be correct with regard to the 
fineness (48,000 per inch). If the structure is sufficiently coarse 
(24,000 per inch) to permit a spectrum of (2) and D to pass through 
the marginal zone, then a spectrum of (1) will pass through the centre 
of the objective. Spherical aberration will prevent combination, and 
therefore an image of double the fineness (48,000 per inch) will be 
seen. 
