GLASGOW SOCIETY OF FIELD NATURALISTS. 127 



The magnified image produced by the rays of light after having 

 passed the object-glass and crossed in the focus of the back 

 combination, are viewed by a simple microscope at the upper end 

 of the body called the Eye-piece. 



The Huyghenian eye-piece (fig. 10), commonly used, consists of 

 two plano-convex lenses with their convex sides directed towards 

 the object-glass. The lower larger one is called the field-glass, the 

 upper one the eye-glass. These eye-pieces give a field of from 4 

 to 6 inches diameter. The Huyghenian eye-piece is not achromatic, 

 being under corrected for colour, and to secure achromatism of the 

 microscope ail good object-glasses are so constructed that they are 

 over corrected for colour. 



The Kellner or orthoscopic eye-piece (fig. 11) has a double convex 

 lens placed in the focus of the achromatic eye-glass without the 

 interposition of a diapliragm. It does not give quite as good 

 definition as the ordinary eye-piece, but it yields a large, flat, well- 

 lit-up field of from 12 to 14 inches diameter. I can confidently 

 recommend the Kellner eye-piece No. C, as made by E,oss, but 

 these Kellner eye-pieces should not be bought in pairs. 



To every fii^st-class microscope there belong a number of eye- 

 pieces of various powers, which English makers commonly designate 

 by A, B, C, D, E, and F ; A being the shallowest and least power- 

 ful, F the deepest and most powerful, magnifying diametrically 

 about eight times as much as A. Although a great increase of 

 magnification is obtained by the use of high eye-pieces, these 

 magnifications are not of great value, as the brilliancy with which 

 an object appears illuminated is reduced in inverse ratio to the 

 superficial magnifying power ; thus by the use of eye-piece F, the 

 object appears only l-64th as brightly lit up as under A. In 

 addition to the loss of light, the errors of the object-glass are 

 magnified without gaining any compensation in focal depth. More- 

 over, high eye-pieces, owing to their short foci, are very trying to 

 the eyes. In fact the use of high eye-pieces recommends itself, 

 only when without the trouble of changing the object-glass a 

 large magnification has to be obtained, good definition being only 

 a matter of secondary importance. The magnification of a micro- 

 scope may also be increased without impairing the definition per- 

 ceptibly, especially when shallow eye-pieces are used, by increasing 

 the distance between the object-glass and the eye-piece by means 

 of a draw-tube, which is generally graduated. 



