26 METHODS OF MICROSCOPICAL RESEARCH. 



the unaided eye, you will be able to discern nothing 

 more than a black dust, the various details having to 

 be made out by other means. Then again, with ob- 

 jects so minute as the diatom, Amphipleura pellucida, 

 the object itself is almost invisible to the unassisted 

 eye, to say nothing of the beautiful carvings with 

 which the valves are embellished, and which exact 

 for their elucidation the most perfect lenses with 

 which we are acquainted, and the most accurate 

 manipulation of the illumination. You may, 

 indeed, see the contour of many forms of diatoms 

 without extra optical assistance than that afforded 

 us by nature, but not much more than this, as if the 

 eye is approached too closely the picture falls behind 

 the retina and is lost. 



I have already mentioned the fact that starting 

 with the distance of most distinct vision, continued 

 approach to the eye finally renders the object in- 

 visible, the rays being thrown behind the retina, the 

 mechanism of accommodation being insufficient to 

 produce a curve deep enough to bring the picture 

 to a short conjugate focus. 



This can, however, be done by interposing a lens, 

 or lenses, between the object and the cornea, so that 

 a virtual image of the object is seen. These lenses 

 form either a simple, or a compound microscope. 



