CONSTRUCTION OF COMPOUND MICROSCOPE. 



83 



introduced between the object-glass and the image formed by it. 

 The ordinary purpose of this lens is to change the course of the 

 rays in such a manner, that the image may be formed of dimen- 

 sions not too great for the whole of it to come within the range of 

 the eye-piece ; and as it thus allows more of the object to be seen 

 at once, it is called the field-glass. It is now usually considered, 

 however, 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 "Huyghenian;" having been employed by 

 Huyghens for his telescopes, although without the knowledge of 

 all the advantages which its best construction renders it capable 

 of affording. It consists of two plano-convex lenses (E E and F F, 

 Fig. 12), 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 precision, at half the sum of the focal length of 

 the eye-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. 

 By Huyghens, this arrangement was in- 

 tended merely to diminish the spherical 

 aberration ; but it was subsequently 

 shown by Boscovich, that the chromatic 

 dispersion was also in a great part cor- 

 rected by it. Since the introduction 

 of Achromatic object-glasses for Com- 

 pound 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 coincident), and 

 thus to produce a colorless image. 

 Thus let L M N (Fig. 13), represent 

 the two extreme rays of three pencils, 

 which, without the field-glass, would 

 form a blue image convex to the eye- 



flass, at B B, and a red one at R R ; then, 

 y 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 shorter for blue rays than for red rays, 

 by just the difference in the place of these images, their rays, 

 after refraction by it, enter the eye in a parallel direction, and 



FIG. 13. 



^ 



Section of Huyghenian Eye-piece 

 adapted to over-corrected Achro- 

 matic objectives. 



