CHROMATIC ABERRATION. 



always the case in the microscope, the object be placed much 

 nearer to the lens than its image, the natter side of the lens 

 should be 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. Fi - 30 - 



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 APLANATIC, from two Greek words 

 a (a) and ir\dvri (plane), which signify no straying. 



53. CHROMATIC ABERRATION. 



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 



* See Tract on "Colour." 



109 



