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the Reader, that he hath found, that the Apertures, which Optick Glasses can bear with distinctness, are in about a subduplicate proportion to their Lengths; whereof he tells us he intends to give the reason and demonstration in his Diopticks, which he is now writing, and intends to finish, as soon as his Health will permit. In the mean time, he presents the Reader with a Table of such Apertures; which is here exhibited to the Consideration of the ingenious, there being of this French Book but one Copy, that is known, in England.
| A TABLE of the Apertures of Object-Glasses. The points put to some of these Number denote Fractions. | |||||||||||||||
| Lengths of Glasses. |
For excellent ones. |
For good ones. |
For ordinary ones. |
Lengths of Glasses. |
For excellent ones. |
For good ones. |
For ordinary ones. | ||||||||
| Feet, Inches. | Inch. Lines. | Inch. Lines. | Inch. Lines. | Feet, Inches. | Inch. Lines. | Inch. Lines. | Inch. Lines. | ||||||||
| 4 | 4. | 4 | 3 | 25 | 3 | 4 | 2 | 10 | 2 | 4. | |||||
| 6 | 5. | 5 | 4 | 30 | 3 | 8 | 3 | 2 | 2 | 7. | |||||
| 9 | 7 | 6. | 5 | 35 | 4 | 0 | 3 | 4 | 2 | 10 | |||||
| 1 | 0 | 8. | 7 | 6 | 40 | 4 | 3 | 3 | 7 | 3 | . | ||||
| 1 | 6 | 9 | 8. | 7 | 45 | 4 | 6 | 3 | 10 | 3 | 2 | ||||
| 2 | 0 | 11 | 10 | 8 | 50 | 4 | 9 | 4 | 0 | 3 | 4. | ||||
| 2 | 6 | 1 | 0 | 11 | 9 | 55 | 5 | 0 | 4 | 3 | 3 | 6. | |||
| 3 | 0 | 1 | 1 | 1 | 0 | 10 | 60 | 5 | 2 | 4 | 6 | 3 | 8. | ||
| 3 | 6 | 1 | 2 | 1 | 1 | 11 | 65 | 5 | 4 | 4 | 8 | 3 | 10 | ||
| 4 | 0 | 1 | 4 | 1 | 2 | 1 | 0 | 70 | 5 | 7 | 4 | 10 | 4 | . | |
| 4 | 6 | 1 | 5 | 1 | 3 | 1 | . | 75 | 5 | 9 | 5 | 0 | 4 | 2. | |
| 5 | 0 | 1 | 6 | 1 | 4 | 1 | 1. | 80 | 5 | 11 | 5 | 2 | 4 | 5 | |
| 6 | 1 | 7. | 1 | 5 | 1 | 2 | 90 | 6 | 4 | 5 | 6 | 4 | 7. | ||
| 7 | 1 | 9 | 1 | 6 | 1 | 3 | 100 | 6 | 8 | 5 | 9 | 4 | 10 | ||
| 8 | 1 | 10 | 1 | 8 | 1 | 4 | 120 | 7 | 5 | 6 | 5 | 5 | 3 | ||
| 9 | 1 | 11 | 1 | 9 | 1 | 5 | 150 | 8 | 0 | 7 | 0 | 5 | 11 | ||
| 10 | 2 | 1 | 1 | 10 | 1 | 6 | 200 | 9 | 6 | 8 | 0 | 6 | 9 | ||
| 12 | 2 | 4 | 2 | 0 | 1 | 8 | 250 | 10 | 6 | 9 | 2 | 7 | 8. | ||
| 14 | 2 | 6 | 2 | 2 | 1 | 9. | 300 | 11 | 6 | 10 | 0 | 8 | 5 | ||
| 16 | 2 | 8 | 2 | 4 | 1 | 11. | 350 | 12 | 6. | 10 | 9 | 9 | 0 | ||
| 18 | 2 | 10 | 2 | 6 | 2 | 1 | 400 | 13 | 4 | 11 | 6 | 9 | 8 | ||
| 20 | 3 | 0 | 2 | 7 | 2 | 2. | |||||||||
