Chromatic and Spherical Aberration. By Dr. Boyston-Pigott. 233 
And speaking of irrationality of dispersion, lie says, " if it had 
no existence we might simultaneously unite lights of all species. 
But since the colours are disproportionately dispersed in different 
media, the other colours will in such a case be very nearly but not 
exactly united. A pencil therefore of light refracted through an 
achromatic combination will illuminate a screen with light still 
slightly coloured, and give rise, as we have stated before (Art. 167), 
to a secondary spectrum. A combination of different media achro- 
matic for all kinds of light being thus in general unattainable, it 
is customary to unite rays which are powerfully illuminating and 
also differ much in colour, the rest remaining partially united."* 
Another familiar example of chromatic aberration being merely 
the spherical aberration of a coloured ray is given by Sir John 
Herschel's explanation of the achromatism of the Huyghenian eye- 
piece, t 
Finally, to give examples, the spherical aberration of the blue 
ray is very different from that of the red ray. 
Whenever spherical aberration exists, there also exists for mono- 
chromatic light a least circle of aberration or the smallest ring 
through which all the rays from a given lens pass. 
For the same lens, aperture, and same conditions of focal distance 
or origin of light, considered as a point, the size of this ring is 
directly proportionate to the dispersion of the glass for that par- 
ticular colour. 
When photographs were taken by Dr. Woodward with blue 
light, the spherical aberration of the other coloured rays was entirely 
got rid of. Dr. W oodward would have found a much more blurred 
image with ordinary compound solar light. 
A blurred image or indistinctness can be produced by residuary 
spherical aberration alone. This very aberration varies notably for 
different colours. It may be as well to state that every funda- 
mental expression or calculated formula for determining aberration 
is used only for homogeneous light, that is, light of only one 
refractive index ; but that may be any value found in nature. 
In this way the chromatic aberration of a given colour is found 
from the spherical : by substituting the refractive index of the said 
colour in the formula for the spherical aberration. 
It is quite plain, to common sense, that the spherical curvature 
of a lens must produce spherical aberration whatever homogeneous 
coloured ray shall be transmitted through it. So that all chromatic 
aberration does involve spherical aberration : and is identical with it 
so far, that every coloured ray on passing through spherical surfaces 
of refraction {as in an ordinary lens) actually produces its own 
* Page 160, Parkinson's ' Optics,' 2nd edition, 
t See Herschel on the Telescope, page 56. 
