THE HUYGHENIAN EYE-PIECE. 



The lower lens is called the field-glass, and the upper 

 one the eye-glass, while a circular stop or diaphragm is 

 placed nearly midway between the two. 



The practical optician Gundlach has stated that the 

 correction afforded by the Huyghenian eye-piece is not 

 a complete one ; for at the point 

 where the spherical aberration 

 is entirely corrected, the chro- 

 matic has not completely disap- 

 peared. This even at the most 

 favourable interval between the 

 two lenses. 



This eye-piece was first em- 

 ployed by Huyghens for his 

 telescopes, in order to diminish 

 spherical aberration and to in- 

 crease the size of the field. An 

 elaborate dissertation upon it has 

 been published by Mr. Varley in 

 the ' Transactions of the Society 

 of Arts,' vol. li., to which the 

 student is referred. It is often 

 called the negative eye-piece, on account of its correcting 

 the positive aberrations of the objective. 



In Fig. 26 is shown a section of a deep eye-piece the 

 Huyghenian C in which it will be seen that the lenses 

 possess deeper curvature than in the A, while the diaphragm 

 is more contracted, and the aperture in the cap covering 

 the eye-glass is very small indeed ; the C eye-piece gives 

 about double the amplification of the A. 



There is another kind of ocular in occasional use, called 

 Ramsden's positive eye-piece ; it is formed of two plano- 

 convex lenses, but the curvature of the field-glass is turned 

 towards the eye instead of towards the object, as in the 



FIG. 25. 



