139 



LENT. 



LEPRA. 



190 



and assuming A to stand for 



1 



the longitudinal aberration is 



I? or - A ]. 



~8 F s F ' 2(1-W) 2 ' F ' 



which must be algebraically applied to the value of v (corrected for 

 thickness). The latitudinal aberration is 



A v i/ 3 3 A w 3 



The distance of the least circle of aberration from the corrected 

 focus is 



3A 



y* 



32 F' F ' 8 (1 -w) F 



and the diameter of the same circle is 



__ _ 



16 F 



_ _ 



8"(i-WJ F"-' 



When two or more lenses are placed close together, in finding the 

 approximate focal distance, uncorrected for the thicknesses, they may 

 be considered as one lens, whose focal distance has a reciprocal equal 

 to the sum of the reciprocals of the focal distances of the component 

 lenses. Sir J. Uerschel has proposed to call the reciprocal of the focal 

 distance the power of a lens ; in which case it would be said that the 

 power of a compound lens is equal to the sum of the powers of the 

 simple lenses. 



For more complicated cases, see the work of Mr. Coddington, 

 already cited. 



Hitherto we have supposed the surfaces of the lens to be either 

 plane or spherical, as is usually the case. Lenses are, however, some- 

 times marie with cylindrical surfaces. If the two surfaces of a thiu 

 lens are portions of cylinders of equal radii, with their axes perpen- 

 dicular to each other, and both surfaces are convex or both concave, 

 the lens will have the same general effect as an equi-convex or 

 equi-concave lens, the radius of the spherical surfaces of which is 

 double that of the cylindrical surfaces of the former lens. Such 

 cylindrical lenses may sometimes be advantageously employed instead 

 of spherical lenses. Thus, when a large lens is held at some distance 

 from the eye, and used for viewing an engraving or photograph, the 

 distortion with a cylindrical lens is of a less unpleasant character than 

 with a spherical lens. 



But there is a further use of lenses with one or both surfaces 

 cylindrical in correcting a defect of the eye, by no means uncommon, 

 which consists in the length of sight being, so to speak, different in 

 different planes passing through the axis of the eye, and which some- 

 times it so great as to cause very serious inconvenience. Its existence 

 is known by directing the eye to the clouds, and holding a small 

 needle-hole in a blackened card at first close to the eye, and then 

 moving it off to arm's length, the eye all the while being adapted for 

 vision of a distant object. The indistinct mirage of the hole ought to 

 remain circular at all distances ; but when this defect exists, the 

 mirage passes successively into two lines in rectangular directions, 

 unless the eye be too long-sighted, in which case the lines are seen by 

 adapting it to vision of a nearer object, or if that be not enough, 

 using an ordinary convex lens. The defect, when it exists, may be 

 corrected by means of spectacles having one surface at least of the 

 lens for the defective eye cylindrical. See papers in the ' Cambridge 

 Philosophical Transactions,' voL 2, p. 267, and vol.^8, p. 493; also 

 'Report of the British Association ' for 1849, p. 10. 



LKNT (in Latin, Quadrayetima), a time of mortification, commemo- 

 rative of the miraculous fasting of our Saviour in the Desert ; used as 

 a preparation for Easter, and beginning on Ash Wednesday. The 

 Saxon term was Ltncten, implying Spring, the season when the day 

 increases in Itnijth, about the commencement of which this fast, which 

 is a moveable one, usually falls : it is in fact the Spring-Fast. 



In the ancient Latin Church, Lent consisted only of thirty-six days : 

 the four additional days were added by Pope Gregory in the 9th 

 centyry. 



Some assert that this fast was first instituted by the Apostles. Such 

 was the opinion of St. Jerome, .fit* Leo, St. Augustine, and others. 

 Tertullian speaks of it as of long standing in his time : though some 

 writers date it as low as the 3rd century. It was first observed in 

 nd by our Saxon ancestors in 640, by order of Ercombert, King 

 of Kent. 



iWhentlejr On Ike Commnn Prayer, 8vo, London, 1741; Brady's 

 i* Calendaria.) 



LENTO (Ital., tloirli/), a term in music equivalent to Largo. 



LEO (the Lion), a constellation of the ZODIAC, which commemorates 

 the Neinican lion killed by Hercules in the mythology of the Greeks. 

 It is surrounded by Ursa Major, Leo Minor, Cancer, Hydra, Sextans, 

 Virgo, and Coma Berenices. A line drawn through the pole star and 





the lowest of the four in the Great Bear (or 7) passes through Deneb 

 (or & Leonis) ; and a line drawn through the bright star Regulus (or o 

 Leonis) of the first magnitude and Deneb passes nearly through 

 Arcturus. The principal stars are as follows : 



Character. 

 A. 



o 



e 



1 

 a 

 ( 

 7 

 t 

 5 



e 



No. in Catalogue 

 No in Catalogue of British 

 of Flamstecd. Association. 



4 

 14 

 17 

 24 

 30 

 32 

 36 

 41 

 47 

 68 

 70 

 74 

 78 

 84 

 93 

 94 



3246 

 3312 

 3331 

 3371 

 3453 

 3459 

 3508 

 3523 

 3609 

 8834 



3848 

 3877 

 3900 

 3990 

 3995 



Magnitude. 

 4 

 34 

 3 

 3 

 II 



3 

 2 

 4 



P 



4 

 4 

 4 

 4 



LEO MINOR, a constellation of Hevelius, surrounded by Ursa 

 Major, Lynx, Cancer, and Leo. Its principal stare are as follows : 



No. in Catalogue 



No. in Catalogue of British 



Character. of Flamsteed. Association. Magnitude. 



I 10 3261 4| 



/ 30 3560 4| 



n 42 3685 4i 



o 46 3728 44 



LEONINE VERSES, a kind of measure much in fashion during 

 the middle ages. It consists properly of the Latin hexameter, or 

 hexameter and pentameter rhymed. No less than ten varieties in the 

 fall of the rhymes are counted ; but that which is by far the most 

 common is when the caesura on the fifth syllable rhymes with the end 

 of the line, as for example : 



" En Rex Edvardus, debacchans ut LeoparduB." 



There is an example of a modern attempt at Leonine verses in 

 Parnell's translation of a passage in the ' Rape of the Lock/ beginning, 



" Et nunc dilectum speculum pro more retectum." 



The rhymes appear universally to be dissyllabic. The classical metre 

 is however not essential. We find in the ancient hymns of the Roman 

 Catholic Church the rhythm of modern versification : 



" Quid sum miser tune dicturus, 

 Quern pratonum rogaturus, 

 Cum vix Justus sit securus ? " 



Or in the famous song of Walter de Mapes, archdeacon of Oxford in 

 the time of Henry II. : 



*' Mihi est propositum in tabcrna mori, 

 Vinum sit appositum moricntia ori, 

 Ut dicant, cum venerint Angelorum chori, 

 Deus tit propitius huic potatori." 



The term is said to be derived from Leoninus, a monk of the 12th 

 century, the reputed inventor of this mode of composition, which how- 

 ever is shown to go back as far at least as the 3rd century. It 

 went out of fashion with the revival of classical learning. 



LEPIDINE. [LEUCOLINE.] 



LEPRA (the Greek word \twpa, scaliness), an affection of the skin, 

 of the order Squama;, or scaly diseases, of Willan and Bateman. It is 

 characterised by an eruption of circular spots of inflamed skin covered 

 with scales; varying from the size of a pin's head to that of a shilling or 

 even a half-crown piece, occasionally mixed with large irregular 

 patches formed by the coalescing of the borders of .several contiguous 

 spots. The scales in this affection possess a peculiar character, by 

 which it is distinguished from pityriasis and psoriasis, other diseases of 

 the same order. From the surface of the inflamed spots a diseased 

 cuticle is produced, which concretes into thickish crusts or scales of a 

 glistening white silvery appearance, and from being secreted more 

 abundantly towards the circumference a rounded and elevated form is 

 given to the outer margin, whilst the centre is left almost or entirely 

 free from scale. The whole is surrounded by a slight areola of redness. 

 In the early stage of the disease, and in the small spots, the inflamed 

 skin, which is slightly raised above the surrounding parts, possesses 

 only a thin scaly covering; and in the larger patches, formed by the 

 coalescence of several spots, the characteristic appearances became 

 somewhat confused ; still, on careful examination, the elevated margin, 

 circular outline, and central free spot may more or less be recognised. 



