PRACTICAL MICROSCOPY. 



Huyghenian. In this eye-piece the focus is obtained in 

 front of the field-glass, while in the Huyghenian ocular the 

 image is formed at the diaphragm, about midway between 

 the field-lens and eye-glass. 



The Ramsden eye-piece was much in use at one time for 

 purposes of micrometry, as it gave an excellent view of the 

 micrometer, free from distortion even to the edges of the 

 field, though the image was slightly coloured. It is still 

 used by Messrs. Ross and Co. for their eye-piece micro- 

 meters. 



FIG. 26. 



FIG. 27. 



Kellner's orthoscopic eye-piece is much employed where 

 a large and flat field is required for use with low powers. 

 A section is shown in Fig. 27, from which the student may 

 gather that the field-glass is doubly convex, and the eye- 

 glass a slightly under-corrected achromatic combination, 

 while the diaphragm is dispensed with altogether. 



In the ordinary or Huyghenian ocular, English opticians 

 designate their power by means of letters, A, B, C, D, E, 

 and F, while some few call their productions by the numbers 

 i, 2, 3, 4, and 5 ; they seem fairly agreed as to what should 

 be the relative degrees of amplification of the A and B 

 eye-pieces, and some with the A, B, and C ; but with 

 higher powers there seems no uniformity, as the following 



