HUYGHEXIAN EYE-PIECE. 



25 



Fig. 15. 



as belonging to the ocular end of the instrument,— the eye-glass 

 and the field-glass being together termed the Eye-piece. Various 

 forms of this Eye-piece have been proposed by different Opticians ; 



and one or another will be 

 preferred according to the 

 purpose for which it may be 

 required. That which it is 

 most advantageous to employ 

 with Achromatic Object- 

 glasses, to the performance 

 of which it is desired to 

 give the greatest possible 

 effect, is termed the Huyghe- 

 nian; having been employed 

 by Huyghens for his tele- 

 scopes, although without the 

 knowledge of all the advan- 

 tages which its best con- 

 struction renders it capable 

 of affording. It consists 

 of two plano-convex lenses 

 (E E and F f, Fig. 14), with 

 their plane sides towards the 

 eye ; these are placed at a 

 distance equal to half the 

 sum of their focal lengths ; 

 or, to speak with more pre- 

 cision, at half the sum of 

 the focal length of the eye- 



Section of Huyyhenian Eye-piece 

 adapted to over-corrected Achromatic 

 objectives. 



glass, and of the distance from the field-glass at which an image of 

 the object-glass would be formed by it. A ' stop' or diaphragm, 

 B b, must be placed between the two lenses, in the visual focus of 

 the Eye-glass, which is, of course, the position wherein the image 

 of the object will be formed by the rays brought into convergence 

 by their passage through the field-glass. — Huyghens devised this 

 arrangement merely to diminish the Spherical aberration ; but it 

 was subsequently shown by Boscovich that the Chromatic dispersion 

 was also in great part corrected by it. Since the introduction of 

 Achromatic Object-glasses for Compound Microscopes, it has been 

 further shown that all error may be avoided by a slight over- 

 correction of these ; so that the blue and red rays may be caused 

 to enter the eye in a parallel direction (though not actually coin- 

 cident), and thus to produce a colourless image. Thus let 1 m n 

 (Fig. 15) represent the two extreme rays of three pencils, which, 

 without the field -glass, would form a blue image convex to the eye- 

 glass at b b, and a red one at R R ; then, by the intervention of 

 the field-glass, a blue image, concave to the eye-glass, is formed at 

 b' b', and a red one at r' r'. As the focus of the Eye-glass is 



