The Photographic and fine art journal . es (in round numbers). This converging pencil isthen refracted by the posterior lens, the positive focal length ofwhich is 18 inches (in round numbers). The effect of this isto diminish the convergency of the rays and bring them to afocus at q, which is further than m from C. The distance AC being one inch, C m is 1 inches, and C q is tlien found in thefollowing way:— Multiply 7X18, and divide the product by their difference;—thatis, divide 126 by 11. This gives C q=llyV inches. Next, let us consider the oblique pencil which proceeds froma distant point
The Photographic and fine art journal . es (in round numbers). This converging pencil isthen refracted by the posterior lens, the positive focal length ofwhich is 18 inches (in round numbers). The effect of this isto diminish the convergency of the rays and bring them to afocus at q, which is further than m from C. The distance AC being one inch, C m is 1 inches, and C q is tlien found in thefollowing way:— Multiply 7X18, and divide the product by their difference;—thatis, divide 126 by 11. This gives C q=llyV inches. Next, let us consider the oblique pencil which proceeds froma distant point P, is incident on the front lens at B,and passes centrically through the backlens at C. Through A, the centre of the front lens, draw a dotted lineA 1, parallel to B. P, and with a A as centre, and and A m asradius, strike an arc of a circle cutting A 1 at 1. Then, A 1equals 8 inches; and the oblique pencil from P will, after pass-ing through the front lens, converge towards the point 1 (asshown by the dotted lines).. Now we come to the pith of the matter. What happens atthe second lens? We have at the second lens an oblique pencil, incident centri-cally, and converging towards 1. Join, therefore, C 1, and pro-duce it to p. Also, with C as centre and C q as radius, strikea circle cutting C p at n. C n is therefore equal to C q. Now, adopting the same formula as in the former case in or-der to find C p, we must must multiply CI by 18 and dividethe product by their difference. What then is the length of C 1 ? In the reply to this query will be seen the great ingenuity ofM. Petzvals arrangement; for it appears that C 1 is greaterthan C m. The proof of this is easy enough. Any two sides of a tri-angle are, together, greater than the third, therefore 1 C and CA are together greater than A 1, and therefore than A away the common part A C, and C 1 is proved to begreater than C m. The actual difference between C 1 and C m in the No 1 lens,with the extreme obli
Size: 1985px × 1258px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1850, bookidphotographic, bookyear1854