CONTENTS. 
PAOI. 
INTRODUCTION 7 
The petrographic microscope 10 
The Gauss equations for paraxial rays 14 
Refraction at a single spherical surface 14 
Refraction through a lens 15 
Principal planes 18 
Nodal planes 18 
The focal length of a lens system of two components 19 
Spherical aberrations 21 
Monochromatic light 21 
Spherical aberration proper (for the principal axis) 21 
Refraction at a single spherical surface 21 
Aplanatic points of a single refracting spherical surface 23 
Spherical aberration in simple lenses 24 
Spherical aberration for object points near the axis (sine condition) 26 
Astigmatism 29 
Curvature of the field (Petzval condition) 30 
Distortion (tangent condition) 30 
Chromatic aberrations 31 
Diaframs in the microscope 34 
Magnification 40 
The field of the microscope 42 
The physical significance of the numerical aperture 42 
Useful and empty magnification in the microscope 47 
The optical elements of the microscope lens system 49 
The objective , 49 
The ocular 52 
The condenser 52 
The Nicol prisms 56 
The petrographic microscope as an accurate measuring device 57 
The adjustment of the petrographic microscope 61 
The stereographic, orthographic, gnomonic and angle projections 63 
Biot's or Fresnel's rule 68 
CHAPTER 1 69 
Color 69 
Pleochroism and absorption 71 
Cleavage and crystal habit 71 
Character of the principal zone 71 
The combination wedge 71 
Optical character of birefracting minerals 74 
Plate perpendicular to the acute bisectrix 78 
Plate perpendicular to an optic axis 78 
Plate parallel to the plane of the optic axes 79 
Dispersion of the optic axes 82 
Dispersion of the bisectrices 82 
CHAPTER II 83 
Refractive indices 83 
The immersion method 92 
The Becke line method 95 
The direct measurement of refractive indices 98 
CHAPTER III 101 
Birefringence 101 
The Newton color scale 108 
Summary 114 
5 
