AND SURGICAL INSTRUMENTS.

North, North Cf.ntrat,, and South Central Galleries.

413

Kin

GDom.]

boning large discs of flint glass is overcome by thePioymejjt of a solid substitute.st ** s substitute is a combination of a resinous sub-es with a very, highly refractive and dispersive(.jj n . a i oil, and its application is extremely simple andl 1(11 5"!'' e ! as well as comparatively inexpensive. From itss con, e ^ eoas na t ure > it would appear that a finer tele-tlian be constructed with lenses of this substance,

'ttatt ‘ enses °f flint-glass; for it has been found aj,| er of extreme difficulty to produce the dense flint-^.Perfectly homogeneous, and free from strife. TheStatute for flint-glass is highly transparent, of a palea. 11(1 w c °l°ur, and is unalterable by light air, moisture,a Pl)l' 11 '?^ erate * leat ’ A uses at about 200 degrees, and isbee 1C< ^ * n a fluifl state between crown-glass lenses. Itn, aI ? es , solid on cooling, and remains so at all at-pur,. eiac temperatures. Its refractive index and dis-bint V ? power allow a shallower system of curves thanrgjji'iBlass usually does, and consequently a better cor-Co “ 0n .°f the spherical aberration. On account of thegla i Sitiou being in perfect contact with the two crown-tefl . enses between which it is put, little light is lost bye as ectl .°n at the four interior surfaces, which is not thee with the ordinary achromatic object-glass,t^h achromatic glass is a compound lens formed ofl; ? ^nds of glass, which act differently upon the rays ofact* 1 t f ausi nitted by them. The effect of their unitedw , .°n is to bring all the rays of light to a common focus,S( , Ca ordinary single lenses will not effect. ObjectsH a trough achromatic lenses are not surrounded witha °? e binges of colour which encircle them when ex-| B by a single lens. Hence the term achromatic.ta ten ° ase ' n fl uest i°n, the disc of flint-glass is substi-by a solid resinous substance.—R. E.]

Denton, J. B., Gray’s Inn Square —Inventor.s keW C1Inens m °d e l or relief mapping, finished and ilieig, “ n ! accompanied by a beam-level, for ascertainingan d distances, with plans and tools elucidating theod of construction.

di s t r - e t mapping is designed for the representation of°Per't- tS w b ere works of drainage and other hydi’aulic.V’elo . 0I1S are contemplated, as the best means of de-a "d d'e natural capabilities of an undulatory surface,st] 'ata ^* s P' a ying the geological formation of the sub-

lulSr' km - ail ’ s draining-level—particularly designed for®bbiU Ws *. nca P a ble of using the spirit-level, but who area J' with the properties of the plumb-bob.

Ca Pabl l0Ve< ^ workman’s level, similar in principle, buttli e sph-it]? reater a djustment, with or without

dpg .

enrose, Francis Cranmer, 4 Trafalgar Square I} e „- —Inventor and Proprietor.

v °lutes terei ^ sci 'ew and sliding helicographs for drawingCurves ’ scr °Uwork, and spirals of various kinds. The°btai ne j a J be drawn on paper by means of an impressiondra\v n i •? ni ‘l |e disc by transfer paper; they may beFj„ ? JP? or with pencil by the sliding helicograph."P°n a ™’ding helicograph. A heavy frame BB' slidesBaide *°°th straight bar AAf which passes through theth e h DG', forming part of the frame. Attached

‘urn s as ^ * s a P°l nt C, on which the instrumentCill ’ r yini» a , centre - Within the frame is a circular ring,°Ue of ,? ax ‘ e of a small wheel or disc, D, fixed toJdth a diameters. This ring, and the disc connected•]'ain e _ ’ .p. a y be placed at any angle with respect to the1 le Wheel l<! . "“ddle part of the frame is hollow, so that! Cre w a t p lest ? on the paper. By means of a gaugebent i s an d the carriage on castors FF', the instru-, ar hs hori» US l e ,, t0 tile proper level. A bar HK, which’defi wit), 0ntall y round K, a point in the frame, is pro-, *Ut may , a PCB'Und-pencil socket, so that the describing' c JUtve 0 / b placed at any point within its range. TheV bole Weiirhf °f ‘be fr' am e is so arranged that itsfc t falls directly upon the wheel, and the edge

Fig. 1.

Penrose’s Registered Helicographs.

of the latter is milled so as to ensure its taking a firmhold of the paper, while the frame slides on the smoothbar. The spiral motion results from the obliquity ofthe axis of the wheel with respect to the smooth bar AA 1when the instrument is made to revolve about the pointC; the rate or pitch of the spiral depending upon theangle which the axis of the wheel makes with the frame.The graduations on the instrument are given on one sidein degrees and on the other in such angles as to makethe longer and shorter radii in one revolution, to eachother as the numbers 2 : 1, 3 : 1 ... 10 : 1, &c. Thepoint C is placed at the same distance from AA 1 as thepoint where the circumference of the wheel touches thepaper. The path of the wheel is the logarithmic spiral,when the instrument is made to revolve about the pointC, and the pen P placed as near as possible to the wheel.To describe a volute (fig. 3), the instrument is made to re-volve about its centre, when the wheel turns round, andcauses the frame to slide uniformly along the bar, until itcarries the describing point as near to the centre as re-quired. The frame being simply brought hack along thebar, all the adjustments remaining unaltered, and thepen placed upon other points, a riband will be formed,preserving an uniformly decreasing thickness all the way.If a parallel line be desired, the pen must he shiftedalong the line of the axis of the wheel, and the lattermade to traverse the same path as it did when the firstline was drawn.

In drawing a volute, or other figure composed ofseveral lines, it will generally be necessary that all thelines should be drawn either towards the eye, or awayfrom it; as owing to a change in the action of the guide-sockets upon the smooth bar, when the motion is re-versed, the angle is slightly affected. _ Th i; s does notimpair the equiangular truth of each individual hue.

Fig. 2 is the screw helicograph; S, the fixed centre ;and N the wheel which works upon the screw.

320 Elliott & Sons, 56 Strand —Manufacturers.

Magazine case of drawing instruments. The same,containing every requisite for drawing circles from the