232 Transactions of the Boyal Microscopical Society. 
II. — On the Identical Characters of Chromatic and Spherical 
Aberration. By Dr. Eoyston-Pigott, M.A., F.E.S. 
(Read he/ore the Eotal Miceoscopical Society, October 6, 1875.) 
DuEiNG- the experience of the last twenty-five years I have had 
opportunities of hearing a numher of persons express opinions about 
spherical and chromatic aberration as though they were totally 
different things. 
To show how identical they are in the nature of things : Take 
an ordinary burning-glass and expose it, first, to blue solar light, 
and secondly, to red.* 
Here, the spherical aberration of white light will now be changed 
into the spherical aberration of the blue and red rays. It will be 
found first of all with the hlue, that the image of the sun is formed 
nearer to the lens ; and secondly, that the red rays form an image 
farther from it. 
This should be well known ; but another point will strike the 
accurate observer, that the size of the image differs in the two cases. 
Lastly, if the marginal rays of the burning-glass be compared 
with those of the centre the image will be formed at very different 
places, showing the spherical aberration of the blue ray ; and the 
same experiment determines the spherical aberration of the red. 
This is for convenience called chromatic aberration, but all the 
time it is really and intrinsically the chromatic spherical aberration. 
For it is the aberration caused by the spherical forms of the lens, 
according as the light is blue or red. 
I have noticed that in recent numbers of the ' M. M. Journal ' 
a confusion has been made as regards spherical and chromatic 
aberration. Thus, one writer makes the somewhat startling 
statement that all chromatic aberration does not involve spherical 
aberration. It may be safely declared, in direct contradiction to 
this, that the aberration of every coloured ray passing through a 
glass lens is wholly due to the spherical curves of the lens. Chro- 
matic aberration, in fact, being really the spherical aberration 
of the particular colour passing. 
There is no other method of finding the chromatic aberration 
of a given coloured ray except by using the refractive index of the 
colour instead of that of mean rays in the expression for spherical 
aberration : mean rays meaning white light. 
It is true that Parkinson says achromatism depends on focal 
length. But he also says that only as many lines of the spectrum 
can be united as there are systems of lenses : so that if there were 
eight lenses of difierent refractive powers, eight fixed lines of the 
spectrum only could be united. 
* Using blue glass or red as a shade. 
