Oct. 2, 1884] 



NA TURE 



543 



masterly essays upon these and others to the Linnean 

 Society. His treatises on the Leguminosae are no less 

 exhaustive and valuable ; and there is not a temperate or 

 tropical region of the globe whose floras have not been 

 largely elucidated by him. It may safely be affirmed that 

 for variety and extent of good work of the kind he had no 

 superior. The distinctive qualities of his descriptions are — 

 scientific accuracy, good arrangement, precision of lan- 

 guage, lucidity, and the discarding of what is superfluous. 

 In these respects he has had no superior since the days 

 of Linnaeus and Robert Brown, and he has left no equal 

 except Asa Gray. 



Of his amiable disposition, and his sterling qualities of 

 head and heart, it is impossible to speak too highly : 

 though cold in manner and excessively shy in dispo- 

 sition, he was the kindest of helpmates and most disin- 

 terested of labourers for others. 



Of recognition by foreign Academies Bentham had his 

 full share, including that of Corresponding Member of 

 the Institute of P'rance. His election into the Royal 

 Society was not till late (1862). It should have been in 

 1829, when he was proposed by R. Brown, and at his 

 recommendation withdrawn, along with other scientific 

 candidates, who thus showed their dissatisfaction at the 

 Society's election of a Royal Duke to the President's 

 chair. He, however, received the Royal Medal of the 

 Society, and in 187S, on the completion of the Australian 

 flora, the Secretary of State for the Colonies, unsolicited, 

 recommended him to Her Majesty for the Companion- 

 ship of the Order of St. Michael and St. George. 



A MODEL LENS FOR USE IN CLASS 

 DEMONS TR A TIONS 

 T N using diagrams or models as aids in teaching, this 

 * question constantly arises — How far may we repre- 

 sent Nature diagrammatically without producing in the 

 mind of a student one-sided and false impressions? I 

 have myself endeavoured to follow this rule : that, if a 

 complicated object or phenomenon is to be studied, we 

 may simplify this, and bring out many salient features, 

 with a diagrammatic representation ; this must, however, 

 only be looked upon as a stepping-stone to a more com- 

 plete study of the object or phenomenon itself. The 



model of a lens to be described I have found of much 

 service in lecturing, antecedent to a demonstration of the 

 passage of luminous rays through actual lenses. 



This model may be constructed out of the simplest 

 materials, and should cost but two or three shillings. It 

 consists of a piece of deal board cut in the shape of the 

 cross-section of a biconvex lens, and fixed to a stand of 

 wood (see diagram). Four small squares of board, x, are 

 fixed in the positions indicated, two on either side of the 

 lens. Glass tubes bent at obtuse angles are fixed to these 

 by staples, and can rotate with them on the screws by 

 means of which the squares are fixed to the lens. Two 

 pieces of string to represent visual rays are then passed 

 through the tubes A A' and B b'. The theory of the use of 



this model will be at once apparent. A ray of light 

 passing through a lens of a given curvature and density 

 will practically (this is not absolutely true) be bent at a 

 given angle, whatever be the direction of the ray, so long 

 as it passes through the same part of the lens. In the 

 model this constant degree of bending is given to the 

 string — representing the ray of light — by the bent tubes. 

 These, rotating on the lens, allow one diagrammatically to 

 represent the rays passing through it in any desired 

 direction. 



Taking the string B b', for example, and holding it at 

 these two points in the two hands, and keeping the string 

 taut, it will be found that in shifting the point B — repre- 

 senting a luminous point — in any direction, B' will shift 

 until it occupies the position of the corresponding focus. 

 By shifting the string it is possible to demonstrate the 

 focal points of parallel, diverging, and converging rays, 

 either parallel to the axis of the lens, or on secondary 

 axes. Then, by using at the same time the string fixed 

 to the other side of the lens A a', the formation of an 

 image may be shown. Grasping with the two hands 

 a and B, an assistant holding a' and B', it will be seen 

 how by this lens an inverted image is produced. Bring 

 the points A and B nearer the lens, keeping them, how- 

 ever, at the same distance apart, and the points A' and b' 

 will recede from the lens and from each other, showing 

 how the image of the nearer object is formed farther away 

 from the lens, and is larger in size. On the other hand, 

 if A and b be pulled away from the lens, a' and b' approxi- 

 mate to it and to one another. 



In working the model the squares should rotate easily, 

 and the strings must always be held taut. For lecture- 

 room purposes the lens should be about two feet high, 

 and the strings may be coloured. On the same principle 

 I have constructed models of other lenses or lens com- 

 binations. John B. Haycraft 



Physiological Laboratory, Mason College, 

 Birmingham 



THE ELECTRICAL EXHIBITION AT 



PHILADELPHIA 



[From a Correspondent] 



TO one who has visited the various electrical exhibitions 

 held in recent years in the chief European cities, 

 the Exhibition now open in the city of Philadelphia might 

 seem a little disappointing from the absence of novelty in 

 the exhibits, though replete with objects of interest for 

 all to whom this class of show is not familiar. As might 

 be expected, the strong point of the Philadelphia show is 

 electric lighting ; and the building — a temporary structure 

 erected close to the depot of the Pennsylvania Railroad 

 Company on the west bank of the Schuylkill — presents 

 both interiorly and exteriorly a blaze of light. The array 

 of dynamo-machines is remarkably complete. Edison 

 leads the way with a variety of machines, including one 

 "Jumbo," of the same type as, but rather smaller than, 

 the pair of machines in use at the Holborn Viaduct 

 lighting station. There are also a large number of 

 Weston's machines, and a very valuable exhibit by the 

 Thomson-Houston Company. The machines of the latter 

 company are quite unique amongst dynamos ; one of 

 them can maintain sixty arc lights on one circuit, though 

 there are but three coils on its armature. Other dynamos 

 are shown by Ball, Hockhausen, Van Depoele, and 

 McTighe. There are several excellent arc lamps, pos- 

 sessing novelty, however, in matters of detail only. A 

 great show of incandescent lamps is made by the Edison 

 Company, and also by the Weston Company. The latter 

 has some remarkable lamps with filaments sixteen inches 

 in length prepared from a new parchment-like substance, 

 of which samples are shown. These lamps are from too 

 to 200 reputed candle power. 



Electric motors are exhibited by several inventors. 



