Optical Construction 51 



the law of refraction, there is a constant ratio for any given 

 two media between the sine of the angle of incidence (being 

 the angle included between the incident ray in the first 

 medium and the perpendicular) and the sine of the angle 

 of refraction, or of the angle included between the ray after 

 refraction and the same perpendicular. The numerical 

 value of this ratio for a ray passing from air into a medium 

 is called the refractive index of the medium. 



Secondary Spectrum. In an achromatic lens the chro- 

 matic aberration is corrected for the brightest (yellow or 

 green) rays of the spectrum, and the pronounced colour 

 shown by uncorrected lenses is in consequence removed. A 

 stricter examination, however, shows that rays of a different 

 colour are not brought to the same focus, for owing to the 

 fact that flint-glass, as compared with crown-glass, disperses 

 the more refrangible rays relatively too much, and the least 

 refrangible relatively too little, a peculiar secondary spectrum 

 results from the achromatic combination, the rays corre- 

 sponding to the brightest apple-green part of the ordinary 

 spectrum being very closely united and focused nearest the 

 combination, whilst the other colours focus at increasing 

 distances in pairs, yellow being united with dark green, 

 orange with blue, red with indigo. The composite effect of 

 these colours is best seen with oblique light, causing dark 

 objects to have apple-green borders on one side and purple 

 ones on the other. 



Semi-apochromatic Correction. In achromatic microscope 

 objectives of the older type, chromatic defects that are worse 

 than the secondary spectrum are caused by spherical aber- 

 ration of the coloured rays, the spherical aberration being 

 corrected for the brightest part of the spectrum only. 

 Objectives made entirely of glass, and therefore showing the 

 secondary spectrum, are called semi-apochromatic when the 

 spherical aberration is corrected practically for all colours. 



Spectrum. A band of colours produced by the splitting 

 up of white light by means of a prism. The order of the 

 colours is : red, orange, yellow, green, blue, indigo, violet. 



42 



