CHAPTER XI. 



/HE USE OF THE EYE-PIECE OR OCULA.R. STOPS. 

 REFLECTIONS. 



THE ocular in ordinary microscopy is an optical system 

 whereby the aerial image produced by the objective is " taken 

 up " and projected, magnified more or less, on the retina of 

 the human eye. The ordinary Huyghenian ocular is made 

 for this sole purpose ; it is frequently made non-achromatic 

 It is therefore not to be expected that an ordinary Huyghen- 

 ian ocular, particularly one not achromatized, should project 

 a perfect, or even a good image on a flat plate perhaps 30 

 inches distant from the spot for which the ocular was intended 

 to work. The writer is, however, bound to accept as a fact in 

 the experience of others what his own experiments have in- 

 variably failed to verify, viz. : that in some cases an ordinary 

 microscope ocular does project on a screen, distant from 

 the ocular from 20 to 40 inches, a true image, and that a 

 photograph tolerably faithful to nature can be made of that 

 projected image in the usual way. Certainly it is conceivable 

 that by some accidental suitability of ocular to objective such 

 a result may be obtained. The writer, therefore, does not 

 gainsay the assertion that photo-micrographs of the highest 

 quality and of difficult objects may be produced by the use of 

 the ordinary achromatic eye-piece, but he does say that he has 

 never produced nor ever seen any photo-micrographs that he 

 could call first-rate obtained by the use of the common eye- 

 piece sold with ordinary microscopes and used for ordinary 

 observation. On the other hand microscopic objectives are 

 not intended for projecting images upon screens several feet 

 distant, but are constructed so that their best image falls 

 somewhere between 6 and 12 inches up to the microscope 

 tube ; and, moreover, that image is intended to be " picked 



