REFRACTION BY CONVEX LENSES. O 



the perpendicular, will undergo no refraction ; the others will be 

 bent from their original course in an increasing degree, in pro- 

 portion as they fall at a distance from the centre of the lens ; and 

 the effect upon the whole will be such, that they will be caused to 

 meet at a point, called the Focus, some distance beyond the centre 

 of curvature. — This effect will not be materially changed by 

 allowing the rays to pass into air again through a plane surface 

 of glass, perpendicular to the axial ray (Fig. 2) ; a lens of this 

 description is called a plano-convex lens, and will hereafter be 

 shown to possess properties which render it very useful in the 

 construction of microscopes. But if, instead of passing through a 

 plane surface, the rays re-enter the air through a second convex 

 surface, turned in the opposite direction, as in a double-convex 

 lens, they will be made to converge still more. This will be 

 readily comprehended, when it is borne in mind that the contrary 

 direction of the second surface, and the contrary direction of its 

 refraction (this being from the denser medium, instead of into it), 

 antagonize each other ; so that the second convex surface exerts 

 an influence on the course of the rays passing through it, which 

 is almost exactly equivalent to that of the first. Hence the focus 

 of a double -convex lens will be at just half the distance, or 

 (as commonly expressed) will be half the length of the focus 

 of a plano-convex lens having the same curvature on one side 

 (Fig. 3). 



4. The distance of the Focus from the Lens will depend not 

 merely upon its degree of curvature, but also upon the refracting 



Fig. 3. 



Parallel rays, falling on a double-convex Lens, brought to a focus 

 in the centre of its sphere of curvature ; conversely, rays diverg- 

 ing from that point rendered parallel. 



power of the substance of which it may be formed ; since the 

 lower the index of refraction, the less will the oblique rays be 

 deflected towards the axial ray, and the more remote will be their 



