Lectures and Illustrated Instructions in Physics, Ophthalmology, Dietetics, Hygiene and Kindred Sciences

Lectures and Illustrated Instructions in Physics, Ophthalmology, Dietetics, Hygiene and Kindred Sciences

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 edition. Excerpt: ...the difference, if any, will be the power of the sphere which will retain the sign of the lens of the highest power; change the sign of the cylinder to the opposite, minus or plus; allow its power to remain the same. reverse the axis 90" from where it was and it will be correctly written. If the sphere should be very strong and the cylinder weak; say, sphere plus 16.00 D. cylinder plus 2.00 D. axis 90, the sphere could be divided. By putting one-half of the total power on one side of the lens; say, about plus 9.00 D. and the balance of the sphere (plus 7.00 D.) on the other side, you have the total of the original (plus 16.00 D.) divided on each side. Then by adding the plus 2.00 D. cylinder power on the sphere of plus 7.00 D. power, there will be plus 9.00 D. on each side in one meridian and the opposite meridian will have plus 9.00 D. on one side and plus 7.00 D. on the other. It should be written + 9.00 D., +7.00 D., +2.00 D. axis 90. Lenses of this kind with the sphere divided on each side, and in compound with cylinder, are called "spherotoric" lenses. Cross cylinders are found by the shadow test on account of the fact that only one meridian of the eye is tested at a time. If they should happen to be the same power in each meridian of the eye of the same kind, either plus or minus, they would be only spheres in effect, as all spheres are cylinders in effect with a right angle axis. When one meridian is considered it is a cylinder in effect. In compound hyperopic or myopic astigmatism each principal meridian would show defect and the lens that neutralized the one of least defect would indicate the power of the sphere. The difference between them would indicate the power of the cylinder which will retain the sign and...show more

Product details

  • Paperback | 26 pages
  • 189 x 246 x 1mm | 68g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236751418
  • 9781236751416