| Name and Model Number | Keuffel & Esser American Optical Dr. Prince Refracting Rule |
| Country of Origin | USA |
| Date of Manufacture | 1920-1950 |
| Front Scales | Age in years, Dioptric Correction |
| Back Scales | Instructions |
| Scale Length | 48 cm (19 inches) |
| Type of Rule | Slide Rule |
| Construction | Solid boxwood or pearwood frame, no slider – scales are logarithmic |
| Area of Use | Technical |
| Comment | Refracting Rule for eye examinations replaced by electronic gadgetry – see JOS-v17-n2-2008-page28 Title: Dr. Prince's Refracting Rule Made by K&E |
| Rarity | RRR |
| Donor | Collection of Clay Castleberry |
| Match Number | 374 |
| Thumbnail (Click Picture for Details) |  |
| In conjunction with our 'donors' we assign a rarity rating to each item, based on the system devised by Herman van Herwijnen |
| (1929 - 2004), renowned Dutch collector, author, and creator of Herman's Archive, a collection of 5,000 slide rule images on CD. |
| • RR = Rare - 1 of 25 collectors has one; it may take a year to find an example in the marketplace. |
| • RRR = Very Rare - only a few known to collectors; may be several years before it comes up for sale. |
| • RRRR = Extremely Rare - a museum piece; unlikely that an example will come up for sale. |