23 6 Theory of Lenjes: [Book I II. 



In the chapter which rivaled generally of refraction, 

 the fubject of LF.NSL . .perflcially con- 



:cij the tlieory oft-. icniains now to be 



invefligated, and from their great importance in the 

 fcience of optics, it will be ntcdfary to fpeak of them 

 fomewhat in dct 



The different forms of lenfes have been already 

 mentioned; but it is necefuiry to premife, that in in- 

 veitigating the properties of a lens, we cdnfidcr its 

 thicknefs as very inconfiderable, and that in every fpe- 

 cies there is a poin", through whieh if a line is drawn 

 in any direction^ and interfered by the furfaces of the 

 lens, a ray refracted by one furface into this line will, 

 after the fecond refraction, emerge parallel to its firft 

 direction. 



Let AI0B0 (Plate XVII. Fig. 24, 25.) reprefcnt 

 a convex or concave lens, the radii of whofe furfaces arc 

 equal, and draw C 1, ci from the centers C, c, paral- 

 lel to each other, and join I /. Suppofe now I / to be 

 a ray of light within the lens refracted by both furfaces 

 at I and i. Since the radii are parallel, the angfes of 

 incidence are equal, and confequently the angles of re- 

 fraction are equal, and the rcfra6ted rays muft make 

 equal angles with the incident ray I /, that is, they mufl 

 be parallel to each other. A ray, therefore, incident 

 on I, and proceeding in the direction I i, will, after re- 

 fraction at ij proceed in a direction parallel to its firft 

 direction. In the fame manner any other ray incident 

 on one furface, and proceeding in die Irne joining two 

 parallel radii, will, after refraction at the feccrid fur- 

 face, emerge parallel to its fiifV direction. But the line 

 joining two parallel radii will always pafs through the 

 fame point m; therefore all rays pafiing through this 

 point m will, after- refraction at the fecond furface, pro- 

 ceed parallel to the direction, which they had before 



the 



