CHAPTER II. 



MECHANICAL AND OPTICAL PRINCIPLES INVOLVED IN THE CONSTRUCTION 



OF THE MICROSCOPE LENSES MODE OP ESTIMATING THEIR POWER, 



ETC. ACHROMATIC LENSES MAGNIFYING POWER WOLLASTON'S 



DOUBLET — CODDINGTON'S LENS — ROSS's SIMPLE AND COMPOUND 

 MICROSCOPES — MICROMETERS, ETC. 



N the construction of the modern microscope optical 

 and mechanical principles of some importance are 

 involved. These principles we now proceed to no- 

 tice, together with the more recent improvements 

 effected in the instrmnent generally. 



The microscope depends for its utility and ope- 

 ration upon concave and convex lenses, and the 

 ' course of the rays of light passing through tb&n. 

 Lenses are usually defined as pieces of glass, or 

 other transparent substances, having their two sur- 

 faces so formed that the rays of light, in passing 

 through them, have their direction changed, and 

 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 obligue direction from 

 one transparent medium to another of a diflFerent density, the direc- 

 tion 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 per- 

 pendicular, and vice veirsd. 



Dr. Arnott remarks : " But for this fact, which to many persons might 

 at first appear a subject of regret, as preventing the distinct vision of 

 objects through all transparent media, light could have been of little 

 utility to man. There could have been neither lenses, as now ; nor any 

 optical instruments, as telescopes and microscopes, of which lenses 



