PLATE I. 



OPTICS or THE COMPOUND MICROSCOPE. l 



FI Upper focal plane of objective. F 2 Lower focal plane of eyepiece. 

 A Optical tube length = distance between FI and F 2 . 

 Oi Object. O 2 Real image in F 2 transposed by collective lens. 

 Oa Real image in eyepiece diaphragm. 



O 4 Virtual image formed at the projection distance C, 250 mm. from EP. 

 1 EP Eye-point. CD Condenser diaphragm. L Mechanical tube length 



(160 mm.), 

 i, 2, 3 Three pencils of parallel light coming from different points of a distant 



illuminant. 



1 From "The Microscopy of Drinking Water" by George C. Whipple. Reproduced through the 

 courtesy of the author and that of the Bausch & Lomb Optical Co. 



Frontispiece 



