MIOKOSOOPE. 



really employed; A the perpendicular drawn from its centre, 

 and A JB, A D, the extreme rays of the incident pencil of 

 light DAB. Then the diameter being 2 C B, the area to 

 which the intensity of vision is proportional will be (2 C B) 2 , 

 and B is evidently the tangent of the angle CAB, which is 

 half the angle of the admitted pencil DAB. 

 Or, if a be used to denote the angular aper- 

 ture, the expression for the intensity is (2 

 tan. i) 2 , which increases so rapidly with the 

 increase of a as to make the loss of light by 

 reflection and absorption of little conse- 

 quence. 



The combination of three lenses ap- 

 proaches, as has been stated, very close to 

 the object; so close, indeed, as to prevent 

 the use of more than three; and this consti- 

 tutes a limit to the improvement of the sim- 

 ple microscope, for it is called a simple microscope, although 

 consisting of three lenses, and although a compound micro- 

 scope may be made of only three or even two lenses; but the 

 different arrangement which gives rise to the term compound 

 will be better understood when that instrument is explained. 



Before we proceed to describe the simple microscope and its 

 appendages, it will be well to explain such other points in refer- 

 ence to the form and materials of lenses as are most likely to 

 be interesting. 



A very useful form of lens was proposed by Dr. Wollaston, 

 and called by him the Periscopic lens. It consisted of two 

 hemispherical lenses, cemented together by their plane faces, 

 having a stop between them to limit the aperture. A similar 

 proposal was made by Mr. Coddington, who, however, executed 

 the project in a better manner, by cutting a groove in a whole 

 sphere, and filling the groove with opaque matter. His lens, 

 which is the well-known Coddington lens, is shown in Fig. 8. 

 It gives a large field of view, which is equally good in all direc- 

 tions, as it is evident that the pencils A A and B B pass through 

 under precisely the same circumstances. Its spherical form has 

 the further advantage of rendering the position in which it is 

 held of comparatively little consequence. It is therefore very 



