The following table gives the sizes and particulars of carbons for various currents that I have found best in actual practice:

CONTINUOUS CURRENT

Amperes. + Carbon _Cored_. - Carbon _Solid_.

7-10 12 mm. 7 mm.

10-15 13 ,, 8 ,, 15-20 16 ,, 10 ,,

ALTERNATING CURRENT

Amperes. Upper Carbon _Cored_. Lower Carbon _Solid_.

7-10 12 mm. 10 mm.

10-15 13 ,, 11 ,, 15-20 16 ,, 13 ,,

CHAPTER VII

THE OPTICAL SYSTEM OF A LANTERN

[Ill.u.s.tration: FIG. 33A.--Optical System of Lantern.]

As previously noted, the essential parts of an Optical Lantern are, in order from rear to front: (1) The illuminant; (2) the condenser; (3) the slide and slide stage; (4) the objective, to which must be added, (5) the body or framework which holds the whole together. Fig. 33 is a diagrammatic representation of the entire optical system and Fig. 33A shows all the various parts _in situ_: A being the illuminant, shown in Fig. 33 {58} as an arc lamp, B the condenser, C the slide stage, and D the objective. The foundation, so to speak, of the whole instrument is of course the slide, which, as made in this country, consists of a square of gla.s.s 3 inches diameter, the slide itself being somewhat less than this on account of the binding, &c.; in making calculations it is usually taken as a 3-inch circle. Slides are usually made by binding together with strips of paper or cloth two such squares, on one of which is the photographic film or painting forming the picture, the other being simply a plain cover gla.s.s placed over the slide surface to protect it, and between the two being placed a paper mask with an aperture of whatever size or shape is required, that of the aforesaid 3-inch circle being usually taken as the standard or normal dimension for this aperture.

The slide being illuminated by one of the various methods discussed in the previous chapters, is focussed on the screen by the objective, which must be selected according to the size of picture required and the distance between lantern and screen.

These points will be gone into later, and also details as to various types of objectives and their respective advantages; but it may be said here that a lantern objective consists usually of a combination of lenses of 2 inches or 2 inches diameter mounted in a rackwork focussing system at a distance from the slide of 6 inches to 18 inches, according to the length of its 'focus.' As our slide is from 3 to 3 inches diameter, it is evident that all the light radiating from this cannot possibly get through the objective unless it is _converged_ upon it, and to do this is the function of the condenser. The following two diagrams, Figs. 34 and 35, will make the matter clear.

S represents our gla.s.s slide of 3 inches clear diameter, R the radiant or illuminant, and L our objective, shown here for the sake of simplicity as a single lens.

The slide is well illuminated by the light emanating from {59} R, but it is obvious that the bulk of this light will never pa.s.s through the lens, and, in fact, only the very centre of the slide will under these circ.u.mstances appear upon the screen at all.

[Ill.u.s.tration: FIG. 34.--Optical System without Condenser.]

[Ill.u.s.tration: FIG. 35.--Action of Condenser.]

What is evidently wanted is to _converge_ these outer rays, or in other words to bend them in so that they also pa.s.s through the objective, and this is the function of the condenser as ill.u.s.trated in Fig. 35. The condenser is here represented also by a single lens, but in practice it also is invariably constructed of two or even three lenses, for both optical and mechanical reasons. It is evident from the above diagrams that the condenser must be somewhat larger in diameter than the slide itself, and condensers for ordinary lantern work are usually 4 inches to 4 inches diameter. The former size {60} will suffice if the condenser is placed very close to the slide, but it is often advisable to leave a little intervening s.p.a.ce, especially if the illuminant is a powerful one, in order to allow any condensation of moisture readily to evaporate and escape. Hence lanterns for long range work (which involve, of course, good illumination) are usually made with condensers of 4 inches diameter. Lantern condensers of to-day usually take one of the two forms shown in Fig. 36, but the exact curve must be left to the manufacturer, as the focus of the condenser must have a definite relation to that of the objective. Taking, however, the design of E, the most common of all, the two lenses should not be exactly similar unless the objective is pretty short in focus, or, in other words, unless the distance of the illuminant on the one hand and that of the objective on the other are approximately equal. If the lantern is intended for long range work, that is equipped with a long focus objective, the front component of the condenser should also be constructed longer in focus (that is to say, with a shallower curve) than the rear one, and it is amazing how careless manufacturers are in this respect. If, as is often the case, the lantern is fitted with several objectives of different foci, it is usually necessary to supply alternative condensers also, or at least to supply an interchangeable front component.

[Ill.u.s.tration: FIG. 36.--Forms of Condensers.]

If the entire condenser is too long in focus, light is lost; if too short, it is impossible to obtain an even disc, as there is invariably a dark patch either in the centre or round the edges.

The mounting of the condenser also varies with different makers; but it must be remembered in any case that it gets {61} extremely hot, especially the back component, and hence the gla.s.s must be mounted _loose_ in its cell, otherwise there is great danger of it cracking. Also the s.p.a.ce between the components should be well ventilated, in order to provide for the escape of moisture which usually at the start of a lantern exhibition is deposited upon the gla.s.s, and should be got rid of before the actual lecture commences.

Even with all care, the back component of a condenser will sometimes crack, though such an accident should be a rare occurrence; and hence a professional operator will usually provide himself with a spare lens, and the condenser should be so constructed that it can readily be changed, and with as little delay as possible.

Condenser lenses as made in this country are usually ground from the gla.s.s known as 'English Crown,' and comparatively rarely crack; but they are very slightly green in colour. French condensers, on the other hand, are whiter, but the gla.s.s is more brittle, and a fracture a more common occurrence. The French variety are (or were before the war) cheaper and generally met with in cheaper instruments. More expensive lanterns are usually fitted with English condensers, as the tinge of green is almost imperceptible, and the advantage as regards greater security pretty considerable.

THE SLIDE CARRIER AND SLIDE STAGE.--Taking still the optical system of the lantern in order from back to front, we now come to the slide, slide carrier, and slide stage. The slide itself has already been described, and the carrier is simply a mechanical contrivance, usually of wood, designed for the purpose of readily changing the pictures and which in its turn fits into the stage of the lantern. It may be asked why, if slides are now always made to a standard size, the slide carrier should not itself be built into the lantern and form the stage; but the answer is, in the first place, that slides of a different size, _i.e._ American or Continental, _may_ be met with, {62} and also that there are various mechanical slides on the market--for example, chromotropes or scientific models, such for instance as are made to ill.u.s.trate the movements of the planetary bodies--and these slides are permanently mounted in wooden frames which could not be put into a carrier. The commonest form of carrier is that known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame with two apertures for the slide, and an outer frame through which this itself slides and which fits the stage of the lantern.

[Ill.u.s.tration: FIG. 37.--Double Sliding Carrier.]

This carrier, as will be seen, allows the next picture to be placed in position in the second aperture while the former one is being projected, and at a signal from the lecturer, the inner frame slides smoothly through the outer, and the slides are thereby changed. This carrier is simple, cheap, and quiet in its action; its one disadvantage is that each alternate slide has to be inserted from opposite sides of the lantern, and unless the operator is fairly tall this almost necessitates an a.s.sistant.

Nevertheless, the carrier is the most popular of any, its other advantages, especially as regards price, being so great. It is usually constructed in such a way that the slide, as it moves out from the central position, automatically rises in its groove in order to facilitate quick removal.

Another pattern deservedly popular is that known as {63} 'Beard's Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all the slides are inserted from the same side, the oncoming slide being pushed _in front_ of its predecessor, and being therefore somewhat out of focus it produces a blur on the screen.

The movement is performed by pushing in a projecting handle, and on withdrawing this the slide which is finished with comes with it, and the finish of the movement presses the new slide back until it is in its proper position and in focus.

[Ill.u.s.tration: FIG. 38.--Beard's Dissolving Carrier.]

The entire action is simpler than it sounds, and the temporary blurring of the image on the screen during the process of changing is supposed to give somewhat the effect of 'Dissolving Views,' and hence the name 'Dissolving Carrier.'

This appliance is three times the price of the 'Double Sliding' pattern, but the fact that it is worked from one side only is a decided advantage, though on the other hand it is not (unless great care is used) quite so silent in its action as the 'Double Sliding' type.

A further modification of this carrier adapts it to take any of the recognised 'foreign' sizes of slides, so that if a few American ones, for instance, are met with among a collection of English manufacture, there is no need to change the carrier. {64}

There are other varieties of carriers on the market which there is no need particularly to describe, such as, for example, carriers fitted with roller curtains to give the effect of a curtain rolling up, magazine carriers to hold twenty-four or more slides and exhibit them in rotation, and other patterns too numerous to mention. Of these the reader must be left to judge for himself, but, generally speaking, _simplicity_ in a carrier is the most important point to be looked for, and complications, however ingenious, should be avoided.

[Ill.u.s.tration: FIG. 39.--Focussing Action of Lens.]

The lantern stage must also receive consideration, but it will be better to discuss it as part of the mechanical construction of the lantern.

THE OBJECTIVE is really the most vital part of a lantern, as the definition of the picture almost entirely depends upon the excellence or otherwise of this lens. This will be obvious at once when it is realised that the objective has to project on to the distant screen a greatly magnified image of the comparatively small lantern slide, and the intending purchaser is strongly advised to economise almost anywhere rather than on this item.

The action of a lens in focussing the image is perhaps best explained by a simple diagram (Fig. 39), from which it will be seen that all the rays proceeding from any one point on the object are re-converged (when the lens is in focus) to a definite point on the image, and the perfection of the picture depends upon the lens performing this function accurately. {65}

The imperfections are chiefly two, viz. those known as chromatic and spherical aberration respectively. Chromatic aberration simply means that all the colours composing the original beam of, say, white light are not equally refracted or converged, and therefore do not meet again at the same spot (the well-known prism or l.u.s.tre effect), and reveals itself by coloured fringes round the edges of the various details in the picture.

[Ill.u.s.tration: FIG. 40.--Achromatic Lens.]

By spherical aberration we mean that the light falling upon the centre of a lens is not brought to a focus at exactly the same spot as the marginal rays, and a general want of definition is the result, usually accompanied also by a want of 'flatness' in the image, that is to say the edges of the picture do not focus at the same time as the centre.

Chromatic aberration is easily cured by using an achromatic or compound lens made by cementing together two lenses of crown and flint gla.s.s respectively, as in Fig. 40.

It will be seen that the flint gla.s.s component by itself is a _concave_ lens and therefore neutralises in part, or in whole, the convex crown lens.

Flint gla.s.s has both greater dispersive power and also greater refractive power than crown gla.s.s, but fortunately not to the same _degree_; hence a compound lens made in this way and with curves carefully worked out may have its chromatic effect entirely neutralised while retaining very considerable refractive or 'focussing' power, and simple achromatic objectives of this type are quite inexpensive.

In lanterns intended for Science demonstration, as distinct from the mere projection of slides, lenses of this pattern are very frequently used, as they will project the latter when required reasonably well, and for the demonstration of {66} experiments or of apparatus on the screen have advantages that need not be discussed here.

For very long focus lenses also they are sometimes employed, as the trouble from spherical aberration is much less apparent with lenses of long focus than with short, and the difference in expense is much more in the former case than in the latter. For short focus lenses, however, as used in moderate-sized halls, they are not good enough, and the type of lens almost universally employed is that known as the 'Petzval' combination (Fig. 41).

[Ill.u.s.tration: FIG. 41.--Petzval Combination.]

This lens really consists of two achromatic combinations, the pair at the front being cemented together, and that at the rear having an air s.p.a.ce between. The combination is so designed that the spherical aberration of the one pair neutralises that of the other, and the result is or should be a lens corrected both for chromatic and spherical aberration.

These lenses, however, vary very much in the perfection of their results, and as they are at present usually imported in bulk from France, the customer does well to insist upon a demonstration of his own particular lantern before acceptance.