Ophthalmic lenses : dioptric formulae for combined cylindrical lenses, the prism-dioptry and other optical papers / by Charles F. Prentice.

  • Prentice, Charles F. (Charles Frederick), 1854-
Date:
1907
Catalogue details

Licence: In copyright

Credit: Ophthalmic lenses : dioptric formulae for combined cylindrical lenses, the prism-dioptry and other optical papers / by Charles F. Prentice. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    55/216 (page 47)
    NUMERALS OF Metric System Dioptries REFRACTION Focal Distances Centimeters Inch System Approximates Focal Distances US. Standard Inches 400. 0.25 1:160 157! 200. 0.50 1:80 78f 133.3 0.75 1:53 52! 100. I. 1:40 39f- 80. 1.25 1:32 31! 66.7 1.50 1:26 26! 57.1 1.75 1:22 50. 2. 1:20 19U 44.4 2.25 I:l8 17! 40. 2.50 1:16 36.4 2.75 1:14 14* 33.3 3- 1:13 13! 30.8 3-25 1:12 12! 28.6 3.50 1:11 Hi 25. 4. 1:10 of 22.2 4.50 1:9 8f 20. 5. 1:8 7f 18.2 5.50 1:7 n 16.7 6. 1:61 6* 15.4 6.50 1:6 6 14.3 7. 1:5s 54 12.5 8. 1:5 4B 11.1 9- 1:4! 4f 10. 10. 1:4 3H 9.1 II. 1:3s 3* 8.3 12. l:3i 3* 7.7 13. 1:3 3* 7.1 14. 1:2| 2H 6-7 15. 2f 6.3 16. I:2| 2* 5.5 18. 1:24 2* 5. 20. 1:2 if! 2.5 40. 1:1 if The above table has been arranged for comparison of the metric with the old system of numbering, in which 1 inch was adopted as the unit. A lens of 10, 20 or 40 inches focus is therefore represented as being *, *, or * of the refraction of the old standard. The focal distances have been calculated upon the basis that 1 meter = 100 centimeters = 39.37 U. S. standard inches, through dividing each of these equivalents by the dioptral numerals. To render a harmony of the numerals of the two systems possible, it is found necessary to neglect slight fractional variations, as shown in the differences between the divisors in the 3rd with the figures of the 4th column. 1 dioptry being placed as equivalent to lenses of 2, 3, or 4 dioptries may be calculated as * = *, * = *, or * = -fa, respec¬ tively, without materially conflicting with the practical demands upon accuracy in a sub¬ stitution of one system of numerals for the other.