180 OPTICAL PRINCIPLES. 



rical aberration is considerably reduced ; and the con- 

 vexities of the lenses in the eye-piece being situated 

 in an opposite direction to that of those in the object- 

 glass, the spherical aberration of the former reverses 

 and so neutralizes that of the latter. Also the under- 

 correction of the field-glass compensates the over- 

 correction of the object-glass the blue rays which 

 are refracted more than the red by the field-glass, 

 being thrown upon the eye-glass nearer its centre, 

 where the refraction is less, and thus the coloured 

 rays become parallel or nearly so on reaching the 

 eye. Moreover the field- glass collects a larger num- 

 ber of rays than the eye-glass could do alone, so that 

 it enlarges the field and increases its brightness. 



In the best object-glasses the aberrations are so 

 well balanced that the mere covering an object with 

 thin glass is sufficient to disturb the balance and 

 render very delicate markings either misty and co- 

 loured or wholly invisible. The effect produced by 

 a plate of glass may be understood by reference to 

 fig. 26, the rays being supposed to emanate from the 

 object at a ; and it is evident that the refraction of 

 the glass so alters the direction of the rays that they 

 will fall upon the lower combination nearer the centre 

 than if the cover were absent, and thus negative 

 aberration is produced. In the best object-glasses, 

 however, this aberration may almost entirely be re- 

 moved, the lower combination being susceptible of 

 approximation by a screw movement to the second or 

 next above it, so that the ascending rays, being able 

 to continue their oblique course through the increased 

 distance between the object and the lower combina- 

 tion, may fall upon the same portions of the latter 

 that they did before the cover was applied. 



POLARIZATION OF LIGHT. In attempting to give a 

 sketch of this curious and difficult subject, we must 

 suppose the reader to be in possession of a natural cry- 

 stal of calcareous spar, and either two NicoPs prisms 



