Optical Construction 49 



Achromatic Correction. — Owing to the relatively greater 

 dispersive power of flint-glass (containing lead or other 

 heavy metals) as compared with crown-glass, it is "possible 

 to produce a combination of a convex lens of crown-glass 

 with a concave lens of flint, which collects rays like a simple 

 convex lens, but which unites two different colours in the 

 same focal point, thus in a great measure correcting the 

 chromatic aberration. 



Aplanatism. — A freedom from spherical aberration (see 

 below). 



Apochromatic Correction. — The highest attainable correc- 

 tion of microscope objectives, comprising the correction of 

 spherical aberration for all colours, and the union of tliree 

 different colours in one focus, thereby eliminating the 

 secondary spectrum. 



Chromatic Aberration. — White light is the composite 

 effect of a continuous range of colours, passing from red, 

 through yellow, green, and blue to violet (see Spectrum). 

 All transparent media have different refractive indices for 

 these different colours, and as a consequence after their 

 passage through a simple lens the rays do not unite at one 

 focal point. The red rays, being the least refrangible and 

 bending to the smallest extent, unite at the farthest distance 

 from the lens ; the orange and green rays unite at points 

 closer to the lens ; while the violet rays come to a focus at 

 a point nearest to the lens. The confusion of different 

 coloured images resulting from this dispersion is termed 

 ' chromatic aberration.' 



Chromatic Over-correction. — A term used when a lens 

 brings yellow or even orange rays to the shortest focus and 

 best correction. 



Chromatic Under-correction. — A term applied to a lens 

 ■when rays towards the blue end of the spectrum are best 

 corrected. Thus a photographic lens is visually under- 

 corrected. 



Diaphragm. — This is generally understood in optical 

 instruments to be a circular opening in a plate that is 



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