Optical tables and data for use of opticians / by Silvanus P. Thompson.
- Silvanus P. Thompson
- Date:
- 1907
Licence: In copyright
Credit: Optical tables and data for use of opticians / by Silvanus P. Thompson. Source: Wellcome Collection.
103/148 page 91
![piano They require the In the case of the lens just calculation of 3 radii of curvature calculated, it is easy to find the radii Fig. 47. Fig. 48. of |curvature. The formula for the power of any lens (see Art. 28) is :— Power = 0* - 1) (-y + 7-V \?1 12/ Now, as each of the component lenses has a flat face one of the two curvatures is = 0, and the formula becomes:— Power = (/a — 1) -» or r = (p. — 1) -r power. The u here is of course the mean refractive index for the particular kind of glass. Referring to the Table [18] of Chance’s glasses we find:— Hard crown, ^ = 1 • 5175, Dense flint, pD = 1 • 6225, whence r for crown lens = 0-5175 -- 4-07 = 0* 127 metre = + 0 inches. r for flint lens = 0-6225 4- - 2-43 = - 0-256 metre = — 10-08 inches. New Achromats.—A new kind of achromatic combination was devised in 1892 by P. Rudolph. This consists in using for the crown-glass positive lens one of the new Jena glasses having a higher refractive index but a lower dispersion than the flint glass of the negative lens. For example, using the glass called “ O 30 ” in Table 16 for the positive lens along with the flint glass called “ O 726 ” in the same Table. These New Achro¬ mats give a flatter field than the old achromats.](https://iiif.wellcomecollection.org/image/b28064732_0103.jp2/full/800%2C/0/default.jpg)
