CHAPTER TT. 



ftTECHANICAI, AND OPTICAL PRINCIPLES INVOLVED IN THE CONSTRtJCTIOH 



OF THE MICE03C0PE — LENSES — MODE OF ESTIMATING THEIP. FOWEB, 



ETC. — ACHROMATIC LENSES — MAGNIFYING POWER — 



WOLLASTON'S DOITBLET — CODDINGTON'S LENS — ROSS'S 



SIMPLE AND COMPOUND MICROSCOPES — MICROMETERS, 



ETC. 



the construction of the modern mi- 

 croscope, optical and mechanical prin- 

 ciples of some importance are involved. 

 These principles we shall briefly ex- 

 plain, together with the more recent 

 improvements effected in the instru- 

 ment generally. •'• 



The microscope depends for its 

 utility and operation upon concave 

 and convex lenses, and the course of 

 the rays of light passing through them. 

 Lenses are usually defined as pieces of 

 glass, or other transparent substances, 

 having their two surfaces 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 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 versd. 



(1) For a full explanation of the laws of optics, and their application to the 

 construction of lenses, the reader is referred to Dr. Bird and Mr. Brooke's 

 " Manu.il of Natural PhilosOi>hy," Professor Potter's " Elementary Treatise on 

 Optics," Sir David Brewster's " Optics," &c. 



