Arquus est Circulus in coelô quadricolor, quem Graeci vocant Irim

A New Latin Dictionary, founded on the translation of Freund's Latin–German Lexicon, edited by E. A. Andrews, LL.D. Revised, enlarged, and in great part rewritten by Charlton T. Lewis, Ph.D. and Charles Short, LL.D., professor of Latin in Columbia College, N.Y. New York, Cincinnati, Chicago, American Book Company; Oxford: at the Clarendon Press. 1879. Page 155:

arcus, ūs, m. (the orthography, arquus (cf. arquatus), is freq. in MSS., like quum for cum, quur for cur, etc.; cf. Freund ad Cic. Mil. p. 31 sq. Thus Charis. p. 92 P. upon Cic. N. D. 3, 20, 51, reads arcuis; Prisc. p. 712 P. arci; and Non. p. 425, 5, upon Lucr. 6, 526, arqui; but the distinction which the latter gram. points out (arcus suspensus fornix appellatur; arquus non nisi qui in caelo apparet, quam Irim poëtae dixerunt) does not seem to be well founded. — Abl. plur. never found; acc. to the gram., Don. p. 1751, Diom. p. 285, Prisc. p. 779, Rhem. Palaem. p. 1371 P. al., it was arcubus; so Vulg. 2 Esdr. 4, 13; cf. Rudd. I. p. 104, n. 48. — Gen. sing. arqui, Lucr. 6, 526 Lachm., and Cic. N. D. 3, 20, 51 B. and K. — Dat. arcu, Sil. 4, 18. — Nom. plur. ARCVVS, Corp. Inscr. V. 85; Inscr. Henz. 5313: arci, Varr. ap. Non. p. 77, 12. — Acc. ARCOS, Corp. Inscr. II. 3420. — Fem., Enn. ap. Prisc. p. 712 P.; cf. id. 658 P.; and Serv. ad Verg. 6, 610, says that Catull. and others used it as fem.; v. Neue, Formenl. I. p. 679) [cf. Sanscr. arālas = bent, the bent arm, aratnis = Gr. ὠλένη; Lat. ulna; Germ. Elbogen; Engl. elbow. Curt.], prop., something bent; hence, I. A bow (syn. cornu). A. For shooting: intendit crinitus Apollo Arcum auratum, Enn. ap. Cic. Ac. 2, 28, 89 (Trag. v. 54 Müll.): arcus intentus in aliquem, Cic. Sest. 7: haec cernens arcum intendebat Apollo Desuper, Verg. A. 8, 704; 9, 665; so Vulg. Psa. 10, 3; 36, 14: arcum tendere, ib. 3 Reg. 22, 34; ib. 4 Reg. 9, 24: adductus, Verg. A. 5, 507: remissus, Hor. C. 3, 27, 67: arcum dirigere in aliquem, Pers. 3, 60: quom arcum et pharetram mi et sagittas sumpsero, Plaut. Trin. 3, 2, 98; so, arcum suscitare, Vulg. Hab. 3, 9 et saep. — B. The rainbow (fully: pluvius arcus, v. infra, II.), Enn. ap. Prisc. p. 712 P. (Ann. v. 393 Vahl.): Tum color in nigris existit nubibus arqui, * Lucr. 6, 526 Lachm.: arcus ipse ex nubibus efficitur quodam modo coloratis, Cic. N. D. 3, 20, 51: ceu nubibus arcus Mille jacit varios adverso sole colores, Verg. A. 5, 88 Rib.; so Ov. M. 6, 63; 11, 632; 14, 838: pluvius describitur arcus, Hor. A. P. 18; Liv. 30, 2; 41, 21; Plin. 18, 35, 80, § 353; Sen. Q. N. 1, 5 and 6: arcum meum ponam in nubibus, Vulg. Gen. 9, 13 sqq. (in Vulg. Apoc. 4, 3; 10, 1, iris, q. v.) al. — C. A bow or arch in building, a vault, arch, triumphal arch, etc.: efficiens humilem lapidum compagibus arcum, Ov. M. 3, 30; 3, 160; Juv. 3, 11; Suet. Ner. 25: marmoreus arcus, id. Claud. 1; so id. ib. 11; id. Dom. 13; cf. Plin. Pan. 59, 2 Schwarz. — II. Transf. A. Poet. or in post-Aug. prose, any thing arched or curved like a bow; of the breaking of waves: niger arcus aquarum, Ov. M. 11, 568. — Of the windings of a serpent: immensos saltu sinuatur in arcus, Ov. M. 3, 42. — Of a curve in flight: dea se paribus per cælum sustulit alis Ingentemque fugâ secuit sub nubibus arcum, Verg. A. 5, 658. — Of the curving or bendings of a bay: sinus curvos falcatus in arcus, Ov. M. 11, 229 (cf.: inque sinus scindit sese unda reductos, Verg. A. 1, 161). — Of a harbor: Portus ab Euroo fluctu curvatus in arcum, Verg. A. 3, 533. — Of boughs of trees, Verg. G. 2, 26 et saep. — Of the back of a chair, Tac. A. 15, 57. — B. The mathematical arc, Sen. Q. N. 1, 10; Col. 5, 2, 9. — Hence, of the five parallel circles of the globe which bound the zones (or perhaps rather, the zones themselves): via quinque per arcus, Ov. M. 2, 129.

• The Biblical reference to 2 Esdra (= Book of Nehemiah) 4:13 refers to this passage in the Vulgate: "statui in loco post murum per circuitum populum in ordinem cum gladiis suis, et lanceis, et arcubus"; which the King James Version renders in English as as "therefore set I in the lower places behind the wall, and on the higher places, I even set the people after their families with their swords, their spears, and their bows".

 

Ausonii Popmae Frisii De Differentiis Verborum libri IV, et De Usu Antiquae Locutionis libri duo, nunc primum in Germania magno iuventutis bono editi, curante Bartholomeo Musculo, JC. Cum praefatione clarissimi viri Dn. Friderici Taubmani, humanitatis in Academia Wittenbergensis professoris. Editio IV. correctior. M.DC.XXX.V. Marpurgi, typis & sumptibus Nicolai Hampelii, Academ. Typogr. Page 42:

Arcus omnis suspensus Fornix appellatur ab arcendô, quòd se ipsum contineat. Arquus est Circulus in coelô quadricolor, quem Graeci vocant Irim. (m). Varro Lib.4. de Vita Populi Rom. Quòd Arci, quos summô opere fecerat, fessi pondere, diu facti celeriter corruissent. Lucret. (n) Tùm color in nigris existit nubibus Arqui.

Sed haec Differentia à Noniô et Orthographis tradita parum mihi quidem probatur. Nam vetus Scriptura fuit Arquus, vt Oquulus, posteà factum Arcus et Oculus, autore Priscianô. (o) Nec est necesse diversas eiusdem vocis significationes Scripturâ decernere, praesertim ubi eadem origo, ut in his . Vtrumque enim dicitur a rotundâ Formâ.

(m) Arcum facit Arcuarius, & tunc est Machinula, qua sagittas emittimus. Arcum arcuat Murarius, & tunc est Fornix in aedificiis. Sed Arcus pluvius in coelo est pulchrè arcuatus. 

(n) Lib.6. (& non Libro i. uti in caeteris omnibus hujus Libri Editionibus exstat.) v. 536.

(o) Rectè itaque in nonnulis Lucretii Editionibus; praeprimis verò in ista, quae Lugduni ap. Seb. Gryphium An. Chr. M.D.XL. in form. 85 prodiit, legitur pro Arqui, Arci.

Page 299, quoting the Differentiae Dictionum of Nonius Marcellus:

Arcus suspensus fornix appellatur. Arquus non nisi qui in coelo apparet, quem Irim poëtae dixerunt. Unde & Arquati dicuntur, quibus color & oculi virent, quasi in arqui similitudinem.

• The only notice I could find about Ausonius van Popma is in the German Wikipedia: Ausonius von Poma [sic] (1563–1613), friesischer Klassischer Philologe und Jurist.
• Nonius Marcellus was a Roman antiquarian of the same kind as the better known Aulus Gellius. Born in Thubursicum in Numidia, he florished in the first half of the third century, or so we believe; he may have lived as early as the end of the second century, or as late as the second half of the fourth. His importance consists in that he left a large compilation of quotations, titled De compendiosa doctrina, which was picked up, quoted and praised by Priscian in the fifth century; and Priscian's Institutiones Grammaticae was the Latin grammar textbook throughout the Middle Ages.

Ancient Numidia, highlighting the environs of Thubursicum. From the 1871 edition of Samuel Butler's Atlas of Antient Geography, with maps engraved by S. Hall and Edwd. Weller; available on Archive.org. (That's Samuel Butler FRS, the bishop and distinguished classicist, not the poet nor the novelist.)