OPTICAL PRINCIPLES OF THE MICROSCOPE. 



rious forms of this Eye- FITS. is. 



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 advan- 

 tageous to employ with 

 Achromatic object-glass- 

 es, to the performance of 

 which it is desired to give 

 the greatest possible ef- 

 fect, is termed the Huy- 

 ghenian; having been em- 

 ployed by Huyghens for 

 his telescopes, although 

 without the knowledge of 

 all the advantages which 

 its best construction ren- 

 ders it capable of afford- 

 ing. 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 

 length; 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 Ejasram of simplest form 

 f* . , , T of Compound Microscope, 



by it. A 'stop' or dia- 

 phragm, 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 conver- 

 gence 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, ifc has been 

 further shown that nearly 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 co-incident), and 

 thus to produce a colorless image. Thus let N 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 

 TI 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 shorter for blue rays than for red rays by just the diffcr- 



Diagram of complete Com- 

 pound Microscope. 



