CHROMATIC ABERRATION. 
always the case in the microscope, the object he placed much 
nearer to the lens than its image, the flatter side of the lens 
should he presented to the object. 
With such a lens the entire extent of the aberration, the object 
being distant, does not exceed its thickness by more than the 
14th part. If the thickness of the lens be expressed by 1, the 
aberration for a distant object will be 1*07. 
Such a lens is represented in fig. 30, and it will be evident in 
how slight a degree it differs from a plano-convex lens. It may 
therefore be expected that its aberration cannot differ much from 
that of the latter form of lens, which has the advantage of being 
much more easily worked. It is accordingly found by calculation 
that the aberration of a plano-convex exceeds that of a lens 
of the above form, in the proportion of 27 to 25, or 
something less than a twelfth. 
If a plano-convex be used the flat side should be 
presented to the object if it be near, and the convex 
side if it be distant. 
52. Lenses, or combinations of lens, which thus 
practically efface the effects of spherical aberration 
are said to be aplaxatic, from two Greek words 
a (a) and irAduri (plane), which signify no straying, 
53. CHEOMATIC ABEEEATIOX. 
It has been already shown in a former number of 
this “ Museum,” that solar light is a compound prin¬ 
ciple, consisting of several component lights differing 
one from another as well in colour as in their sus¬ 
ceptibility of refraction, and that the colours of all 
natural objects arise from their peculiar properties 
of reflecting light, red objects being those which reflect red 
light, blue those which reflect blue light, and so on, a white 
object being one which reflects indifferently lights of all colours, 
and a black object one which reflects no light. 
54. White light is composed of lights of various tints, varying 
from red to violet in the following order: red, orange, yellow, 
green, blue, indigo, and violet, each colour being less refrangible 
than that which follows it. 
55. Coloured lights may be also more or less compounded; 
thus, various tints of orange may be produced by the combina¬ 
tion of reds and yellows, tints of green by the combination of 
yellows and blues, and so on.* 
56. This being understood, let us suppose an object illuminated 
Fig. 30. 
L 
L 
* See Tract on “Colour.” 
109 
