LIST OF ILLUSTRATIONS. 



The author extends grateful acknowledgments to the opticians and others who 

 have loaned cuts for this edition. The source of each figure is given when bor- 

 rowed. The other figures were drawn expressly for this work by Mrs. Gage. The 

 frontispiece was drawn by Mr. Gtitsell, of the University Art Department. 



Fig. Page. 



Frontispiece 



1-9. The principal axis and center of various lenses 2 



io-ii. Principal focus with converging and diverging lenses 3 



12. Chromatic aberration 4 



13. Spherical aberration 4 



14-15. Real and virtual image with convex lenses 5 



16. Simple microscope and eye of observer 6 



17. Tripod magnifier 7 



18. Achromatic triplet The Bausch & Lomb Opt. Co,) 7 



19. Lens-holder (The Bausch & Lomb Opt. Co. ) 8 



20. Dissecting microscope (The Bausch & Lomb Opt. Co. ) 9 



21. Principle of the compound microscope 10 



22. Dry objective 11 



23. Immersion objective 12 



24. Tube-length 15 



25. Tube-length when nose piece and ocular micrometer are used (Zeiss' cat- 



alog, No. 30) 16 



26. Angular aperture 17 



27-29. Dry and immersion objectives (Ellenberger) 18 



30. Section of Huygeuian ocular for eye-point 22 



31. Compensation oculars (Zeiss' catalog, No. 30) 24 



32. Projection oculars (Zeiss' catalog, No. 30) 25 



33-34. Ocular micrometer with movable scale (Bausch & Lomb Opt. Co.) . . 25 



35. Ocular screw micrometer (Zeiss' catalog, No. 30) 26 



36. Triple nose-piece or revolver (Queen & Co.) 27 



37. Size of field with various objectives and oculars 29 



38. Principle of the simple microscope (Fig. 16 repeated) 31 



39-40. Dry and immersion objectives (Figs. 22-23 repeated) 34 



41. Achromatic condenser (Zeiss' catalog, No. 30) 41 



42-43. Image of diaphragm in centering 42 



44-45. Centering the source of illumination on the object 43 



46-47, Aperture of condenser (from Nelson) 43 



48-51. Abbe condenser, central, oblique and dark ground illumination . , . . 47 



52. Lamp and bull'9 eye condenser 49 



53-55. Refraction diagrams (from Carpenter-Dallinger) 50 



56. Aberration produced by the cover-glass (Ross) 53 



