34 THE ELEMENTS OF VEGETABLE HISTOLOGY 



Fig. 18. — Diagram Showing Path of Light Rays. 



Fi f Upper focal plane of objective. Fz, Lower focal plane of eyepiece. A, Optical 

 tube length = distance between Fi and Fv. Oi, Object. O2, Real image in Fz trans- 

 posed by the collective lens, to 03, Real image in eyepiece diaphragm. Oi, Virtual 

 image formed at the projection distance 0, 250 mm. from EF, Eyepoint. CD, Con- 

 denser diaphragm. L, Mechanical tube length (160 mm.)- 1, 2, 3, Three pencils 

 of parallel light coming from different points of a distant illuminant, for instance, 

 a white cloud, which illuminate three different points of the object. 



