COMPOUND OBJECT-PIECES. 



pendently of other objections ; great powers, therefore, have been 

 obtained by combining several 

 achromatic lenses in the same 

 object-piece, so that the rays 

 proceeding from the object are 

 successively refracted by each 

 of them, and the image sub- 

 mitted to the eye-glass is the 

 result of the whole. 



The optical effect of such a 

 combination will be more 

 clearly understood by refer- 

 ence to fig. 11, where L L, L' i/, 

 and L" L", represent a com- 

 bination of three achromatic 

 object-glasses. Let o o be 

 the object, placed a little 

 within the focus f of the lens 

 L L. The image of o o, pro- 

 duced by L L, would then be 

 an imaginary one in the posi- 

 tion 1 1 ; (see Tract on * * Optical 

 Images," 35, et seq.). After 

 passing through L L, the rays, 

 therefore, fall upon if I/, as if 

 they diverged from the several 

 points of the image 1 1, which 

 may, therefore, be considered 

 as an object placed before the 

 lens L' if. Let f be the focus 

 of L' if ; the image of 1 1 pro- 

 duced by if if will therefore be 

 imaginary, and will be at i' i' ; 

 the rays after passing through 

 if L' will fall upon L" L", as if 

 they diverged from the several 

 points of i' i'. This image i' i', 

 therefore, may be considered 

 as an object placed before the 

 lens L" L". Let/ ff be the focus 

 of this lens ; the image of i' i' 

 produced by L" L" will then be 

 i" i", and will be real ; this will 

 then, in fact, be the image 

 transmitted to the eye-piece. 



21 



