76:
20:
960:
would be used for this, but a lens clock can be used if a dial thickness gauge is not available. To do this, the contact lens is placed concave side up on a table or other hard surface. The lens clock is then brought down on it such that the center prong contacts the lens as close to its center as
550:
with an index of 1.7 is measured with a lens clock calibrated for crown glass with an index of 1.523. For this particular lens, the lens clock gives surface powers of −3.0 and −7.0 diopters (dpt). Because the clock is calibrated for a different refractive index the optical power of the lens is
71:
The lens clock has three pointed probes that make contact with the surface of the lens. The outer two probes are fixed while the center one moves, retracting as the instrument is pressed down on the lens's surface. As the probe retracts, the hand on the face of the dial turns by an amount
931:
838:
742:
649:
332:
Measuring both sides of the lens and adding the surface powers together gives the approximate optical power of the whole lens. (This approximation relies on the assumption that the lens is relatively
183:
532:
418:
936:
Finally, if the lens is thin the powers of each surface can be added to give the approximate optical power of the whole lens: −13.4 diopters. The actual power, as read by a
329:
surface, with a refractive index of 1.523. If the lens is made of some other material, the reading must be adjusted to correct for the difference in refractive index.
472:
272:
101:
979:
441:
365:
316:
296:
252:
226:
206:
846:
753:
657:
564:
1125:
981:
in the formula above, and can be calculated from the optical power reading, if the distance between the outer prongs is known.
1120:
345:
109:
555:
the sum of the surface powers given by the clock. The optical power of the lens is instead obtained as follows:
480:
447:, regardless of the actual index of the lens being measured. If the lens is made of glass with some other index
373:
957:
952:
A lens clock can also be used to estimate the thickness of thin objects, such as a hard or gas-permeable
995:
326:
1015:
961:
possible, and the outer prongs rest on the table. The thickness of the lens is then the sagitta
367:
of the surface can be obtained from the optical power given by the lens clock using the formula
60:
450:
257:
86:
1025:
1000:
1100:
964:
543:
426:
350:
301:
281:
237:
211:
191:
36:
32:
1091:
History of the Royal
Society From Its Institution to the End of the Eighteenth Century
1114:
926:{\displaystyle \phi _{2}={(1.7-1) \over -0.0747\ \mathrm {m} }=-9.37\ \mathrm {dpt} }
52:
40:
833:{\displaystyle \phi _{1}={(1.7-1) \over -0.174\ \mathrm {m} }=-4.02\ \mathrm {dpt} }
1064:
The Royal
Society’s role in the diffusion of information in the seventeenth century
953:
737:{\displaystyle R_{2}={(1.523-1) \over -7.0\ \mathrm {dpt} }=-0.0747\ \mathrm {m} }
644:{\displaystyle R_{1}={(1.523-1) \over -3.0\ \mathrm {dpt} }=-0.174\ \mathrm {m} }
990:
547:
44:
1049:
1035:
941:
937:
1073:
Promoting experimental learning: Experiment and the Royal
Society 1660–1727
1066:. Notes and Records of the Royal Society of London. Royal Society of London.
1020:
333:
48:
1010:
75:
1005:
275:
230:
56:
19:
1030:
319:
74:
18:
254:
is the horizontal separation of the outer probes. To calculate
474:, the true optical power of the surface can be obtained using
325:
A typical lens clock is calibrated to display the power of a
59:, assuming the lens is made of a material with a particular
1082:
Physics at the Royal
Society during Newton’s Presidency
747:
Next, the optical powers of each surface are obtained:
1084:. Los Angeles: William Andrews Clark Memorial Library.
967:
849:
756:
660:
567:
483:
453:
429:
376:
353:
304:
284:
260:
240:
214:
194:
112:
89:
1099:
973:
925:
832:
736:
643:
526:
466:
435:
412:
359:
310:
290:
266:
246:
220:
200:
177:
95:
79:Use and measurements using a calibrated lens clock
1106:. New Brunswick, N.J.: Rutger’s University Press.
1075:. Cambridge, England: Cambridge University Press.
178:{\displaystyle \phi ={2(n-1)s \over (D/2)^{2}},}
8:
558:First, the radii of curvature are obtained:
944:, might differ by as much as 0.1 diopters.
234:) between the center and outer probes, and
1102:Newton’s Optical Writings: A Guided Study
527:{\displaystyle \phi ={(n_{2}-1) \over R}.}
966:
912:
892:
863:
854:
848:
819:
799:
770:
761:
755:
729:
703:
674:
665:
659:
636:
610:
581:
572:
566:
500:
490:
482:
458:
452:
428:
383:
375:
352:
303:
283:
259:
239:
213:
208:is the index of refraction of the glass,
193:
163:
151:
119:
111:
88:
51:of a surface, but gives the result as an
1056:Heath A Treatise on Geometrical Optics.
538:Example—correcting for refractive index
445:for which the lens clock is calibrated
413:{\displaystyle R={(n-1) \over \phi },}
16:Mechanical device for measuring lenses
7:
43:. It is a specialized version of a
919:
916:
913:
893:
826:
823:
820:
800:
730:
710:
707:
704:
637:
617:
614:
611:
14:
1051:A Treatise on Geometrical Optics
1054:. Cambridge University Press.
878:
866:
785:
773:
689:
677:
596:
584:
512:
493:
398:
386:
160:
145:
137:
125:
72:proportional to the distance.
1:
1093:. London: The Royal Society.
103:of the surface is given by
47:. A lens clock measures the
35:that is used to measure the
443:is the index of refraction
1142:
1098:Sepper, Dennis L. (1994).
956:. Ideally, a contact lens
228:is the vertical distance (
1048:Heath, Robert S. (1887).
1080:Heilbron, J. L. (1983).
1126:Dimensional instruments
975:
927:
834:
738:
645:
528:
468:
437:
414:
361:
312:
292:
268:
248:
222:
202:
179:
97:
80:
24:
996:Eyeglass prescription
976:
928:
835:
739:
646:
529:
469:
467:{\displaystyle n_{2}}
438:
415:
362:
318:must be specified in
313:
293:
269:
267:{\displaystyle \phi }
249:
223:
203:
180:
98:
96:{\displaystyle \phi }
78:
22:
1121:Ophthalmic equipment
965:
958:dial thickness gauge
948:Estimating thickness
847:
754:
658:
565:
481:
451:
427:
374:
351:
302:
282:
258:
238:
212:
192:
110:
87:
1089:Thomas, T. (1812).
1071:Hall, M.B. (1991).
1062:Hall, M.B. (1975).
346:radius of curvature
340:Radius of curvature
1016:George Ravenscroft
971:
923:
830:
734:
641:
524:
464:
433:
410:
357:
308:
288:
264:
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218:
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175:
93:
83:The optical power
81:
25:
974:{\displaystyle s}
911:
898:
891:
818:
805:
798:
728:
715:
702:
635:
622:
609:
519:
436:{\displaystyle n}
405:
360:{\displaystyle R}
311:{\displaystyle D}
291:{\displaystyle s}
247:{\displaystyle D}
221:{\displaystyle s}
201:{\displaystyle n}
170:
1133:
1107:
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491:
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94:
61:refractive index
31:is a mechanical
1141:
1140:
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1131:
1130:
1111:
1110:
1097:
1088:
1079:
1070:
1061:
1047:
1044:
1026:Vertex (optics)
1001:Corrective lens
987:
963:
962:
950:
882:
865:
850:
845:
844:
789:
772:
757:
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583:
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17:
12:
11:
5:
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769:
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719:
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673:
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664:
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629:
626:
619:
616:
613:
606:
603:
598:
595:
592:
589:
586:
580:
575:
571:
544:biconcave lens
539:
536:
535:
534:
523:
518:
514:
511:
508:
503:
499:
495:
489:
486:
461:
457:
432:
421:
420:
409:
404:
400:
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388:
382:
379:
356:
341:
338:
307:
287:
263:
243:
217:
197:
186:
185:
174:
166:
162:
158:
154:
150:
147:
142:
139:
136:
133:
130:
127:
124:
118:
115:
92:
68:
65:
37:dioptric power
33:dial indicator
15:
13:
10:
9:
6:
4:
3:
2:
1138:
1127:
1124:
1122:
1119:
1118:
1116:
1104:
1103:
1096:
1092:
1087:
1083:
1078:
1074:
1069:
1065:
1060:
1057:
1053:
1052:
1046:
1045:
1041:
1037:
1034:
1032:
1029:
1027:
1024:
1022:
1019:
1017:
1014:
1012:
1009:
1007:
1004:
1002:
999:
997:
994:
992:
989:
988:
984:
982:
968:
959:
955:
947:
945:
943:
939:
906:
903:
900:
886:
883:
875:
872:
869:
860:
855:
851:
843:
842:
813:
810:
807:
793:
790:
782:
779:
776:
767:
762:
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750:
749:
748:
723:
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717:
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671:
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653:
630:
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624:
604:
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573:
569:
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537:
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455:
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430:
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402:
395:
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389:
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368:
354:
347:
339:
337:
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328:
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321:
305:
285:
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261:
241:
233:
232:
215:
195:
172:
164:
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134:
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128:
122:
116:
113:
106:
105:
104:
90:
77:
73:
66:
64:
62:
58:
54:
53:optical power
50:
46:
42:
38:
34:
30:
21:
1101:
1090:
1081:
1072:
1063:
1055:
1050:
954:contact lens
951:
935:
746:
557:
552:
541:
444:
422:
343:
331:
324:
229:
187:
82:
70:
67:How it works
28:
26:
991:Astigmatism
548:flint glass
327:crown glass
45:spherometer
1115:Categories
1042:References
1036:Gear ratio
942:lensometer
938:vertometer
29:lens clock
23:Lens clock
1021:Optometry
904:−
884:−
873:−
852:ϕ
811:−
791:−
780:−
759:ϕ
721:−
695:−
684:−
628:−
602:−
591:−
507:−
485:ϕ
403:ϕ
393:−
262:ϕ
132:−
114:ϕ
91:ϕ
49:curvature
1011:Lapidary
985:See also
546:made of
276:diopters
57:diopters
1006:Galileo
278:, both
231:sagitta
910:
890:
887:0.0747
817:
797:
727:
724:0.0747
701:
634:
608:
423:where
320:meters
188:where
1031:Clock
794:0.174
681:1.523
631:0.174
588:1.523
39:of a
907:9.37
814:4.02
344:The
334:thin
298:and
41:lens
940:or
870:1.7
777:1.7
698:7.0
605:3.0
553:not
336:.)
274:in
55:in
1117::
542:A
322:.
63:.
27:A
969:s
920:t
917:p
914:d
901:=
894:m
879:)
876:1
867:(
861:=
856:2
827:t
824:p
821:d
808:=
801:m
786:)
783:1
774:(
768:=
763:1
731:m
718:=
711:t
708:p
705:d
690:)
687:1
678:(
672:=
667:2
663:R
638:m
625:=
618:t
615:p
612:d
597:)
594:1
585:(
579:=
574:1
570:R
522:.
517:R
513:)
510:1
502:2
498:n
494:(
488:=
460:2
456:n
431:n
408:,
399:)
396:1
390:n
387:(
381:=
378:R
355:R
306:D
286:s
242:D
216:s
196:n
173:,
165:2
161:)
157:2
153:/
149:D
146:(
141:s
138:)
135:1
129:n
126:(
123:2
117:=
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