16 



THE MICROSCOPE. 



CHAPTER II. 



THE FORMATION OF IMAGES 'BY THE ORGAN OF VISION THEORY OF MICRO- 

 SCOPJCAL VISION CHROMATIC AND SPHERICAL ABERRATION OF LENSES - 

 MECHANICAL AND OPTICAL PRINCIPLES INVOLVED IN THE CONSTRUCTION 

 OF THE MICROSCOPE LENSES ACHROMATIC LENSES MAGNIFYING POWER 

 WOLLASTO.V'S DOUBLET CODDINGTON'S LENS EYE-PIECES 

 SIMPLE ANDCOMPOUND MICROSCOPES CONSTRUCTION APERTURE 

 IMMERSION SYSTEM, ETC. 



N the construction of the modern 

 microscope, optical and mechanical 

 principles of some importance are 

 involved. These principles, together 

 with the more recent improvements 

 effected in the instrument generally, 

 I shall proceed to explain. 1 



The microscope depends for its 

 utility and operation upon concave 

 and convex lenses, and the course of 

 rays of light passing through them. 

 Lenses are usually denned as pieces 

 of glass, or other transparent sub- 

 stances, having their two surfaces 

 so formed that the rays of light, in 

 passing through them, have their 

 direction changed, are made to converge or diverge 

 from their original parallelism, or to become parallel 

 after converging or diverging. When a ray of light 

 passes in an oblique direction from one transparent 

 medium to another of a different density, the direction 

 of the ray is changed both on entering and leaving ; 

 this influence is the result of the well-known law of 

 refraction, that a ray of light passing from a rare into 

 a dense medium is refracted towards the perpendicular, 

 and vice versa. 



(1) For an explanation of the optical laws as applied to the microscope 

 see "The Microscope, in Theory and Practice," Nageli and Schwendener, 

 translated and published by Swan Sonnenschein and Allen, 15, Paternoster 

 Sauare. E.G. Also "A Treatise on Optics," by J. Parkinson. 



