The magnifying power of a lens depends upon its 'focus' multiplied by its distance from the screen, and the focus in the case of a simple lens is easily determined by the familiar 'burning-gla.s.s' experiment, that is by focussing an image of the sun upon a piece of paper and measuring accurately {67} the distance the lens must be away to produce the most concentrated spot.

In practice it is sufficiently accurate to focus a distant window, or other luminous object, upon the paper, any error obtained by this method being for ordinary purposes a negligible one.

With a compound lens, such as a 'Petzval' combination, this method does not hold good, as the optical centre of such a lens is not necessarily midway between its two components.

The actual focus can be got pretty approximately by focussing a window or other object as before and measuring the distance from one definite point (say the front edge of one of the lens cells) to the paper, then turning it round and taking a second measurement from the _same_ point, the mean between the two measurements giving the actual focus.

In practice the 'simple equivalent focus,' as it is termed, of a lantern lens is usually determined by measuring the magnification of the image thrown upon the screen, when, by knowing the original size of the slide (a 'standard' slide of 3 inches diameter is usually taken) and the distance between lantern and screen, we get the focus from the following very simple equation:

Diameter of picture on screen Distance between lens and screen (in feet) (in feet) ----------------------------- = --------------------------------- Diameter of slide (in inches) Focus of lens (in inches)

or perhaps more simply still:

Distance between lens Diameter of slide and screen (in feet) (in inches) ------------------------------------------ = Focus of lens in inches; Diameter of picture (in feet)

or, if we know the focus of the lens but want to know how far from the screen we must go to produce a given-sized picture, the formula will be: {68}

Diameter of picture Focus of lens (in feet) (in inches) ------------------------------------ = Distance required (in feet).

Diameter of slide (in inches)

It is handy for the lanternist to remember that, dealing with a standard 3-inch slide, a 6-inch lens will _always_ give a picture whose diameter is _one-half_ the distance from lens to screen, a 12-inch lens half this again or _one-quarter_, and a 9-inch lens half-way between the two.

Bearing these simple figures in mind, the approximate distance can usually be _guessed_ sufficiently near for the first trial, and then the lantern shifted a little nearer or the reverse as required.

The following table may, however, be useful, as showing readily the magnification produced at different distances by lenses of given foci:

+------+---------+--------+--------+--------+--------+--------+--------+

Disc

Focus

Focus

Focus

Focus

Focus

Focus

Focus

wanted

4 in.

6 in.

8 in.

10 in.

12 in.

15 in.

18 in.

+------+---------+--------+--------+--------+--------+--------+--------+

feet.

ft. in.

ft. in.

ft. in.

ft. in.

ft. in.

ft. in.

ft. in.

9

13 6

18 0

24 0

30 0

36 0

45 0

54 0

12

18 0

24 0

32 0

40 0

48 0

60 0

72 0

15

22 6

30 0

40 0

50 0

60 0

75 0

90 0

18

27 0

36 0

48 0

60 0

72 0

90 0

108 0

20

30 0

40 0

53 4

66 8

80 0

100 0

120 0

25

37 6

50 0

66 8

83 4

100 0

125 0

150 0

30

45 0

60 0

80 0

100 0

120 0

150 0

180 0

+------+---------+--------+--------+--------+--------+--------+--------+

THE DIAMETER OF THE OBJECTIVE.--The diameter of the objective must depend to a certain extent upon its focus in the case of a double combination such as a Petzval. These lenses consist, as has already been said, of two achromatic components some distance apart, and for technical considerations, which need not be discussed here, the _distance_ between these components is usually about two-thirds of the focal length. This is not a universal rule, as the lenses of different makers vary a good deal; but it is generally a fact {69} that the longer the focus of the lens the greater is usually the separation between the two lens systems.

The entire lens therefore mounted in its tube resembles a _tunnel_ of varying length according to its focus, and through this tunnel a _cone_ of light rays have to be pa.s.sed. It is plain, therefore, that a lens of long focus, which in practice means a long tube length, must be made also of large diameter, or a portion of the cone will be cut off, with a consequent loss of light.

In practice lenses up to 6 inches focus are usually made of 2 inches diameter, and there is no advantage in a larger size. With a lens of 8 inches focus there is a slight gain in increasing the diameter to 2-3/8 (the next 'standard' size), and lenses of longer focus than this should certainly be 2-3/8 inches up to, say, 12 inches focus, when a lens of 3 inches diameter is preferable. These large lenses are, however, very expensive, both in themselves and also on account of the fact that their weight entails heavy and expensive bra.s.s mounting, and hence lenses up to 14 or 15 inches focus are often supplied in the 2-3/8 size for reasons of economy.

To sum up, _short-range_ lanterns, as they are called, are usually fitted with lenses of 2 inches diameter, and _long-range_ instruments either with 3-inch lenses or the intermediate size of 2-3/8 inches. If a lantern is purchased for either long or short-range work, it is usually fitted with a bra.s.s front for a large lens, and so arranged that a shorter focus lens of 2 inches diameter can easily be interchanged, utilising the same bra.s.s mounting.

Lenses of _variable_ focus have also been designed, in which an additional lens can be added or subtracted to increase or decrease the focal length; but nothing very practical has yet been achieved in this direction, and therefore these 'Omnifocal' lenses have never come into general favour.

Objectives like condensers want cleaning at times, and care must be taken not to scratch the gla.s.s, as the concave lens of each component is of flint gla.s.s, and very soft. A {70} clean chamois leather is the best thing to use, but a soft cloth, or even a handkerchief, may be employed with care.

It is very important that a lens be rea.s.sembled, after cleaning, the correct way, as a single lens reversed would utterly spoil the definition.

The front component is usually balsamed together, and therefore all that is needed is to see that the whole combination is not reversed. In the Petzval system this lens should have its convex const.i.tuent towards the screen (Fig. 41). The back combination is usually loose, and the two lenses are sometimes separated by a thin bra.s.s ring. In the Petzval lens the concave element should be inside, with its concave surface outwards, the deep curve of the other lens should fit into this concavity, and the flatter curve face towards the condenser. One or two makers, however, have introduced a modification of the Petzval system in which the whole of this back combination is reversed, and the exact arrangement should therefore be noted very carefully when taking the lens to pieces.

CHAPTER VIII

THE BODY OF THE LANTERN

We now come to the mechanical construction of the optical lantern, and a great variety of design presents itself, according to price, type (_i.e._ short range or long range), and the individual ideas of the various makers.

Lantern bodies as a rule are now made of metal, although up till quite recently the better cla.s.s instruments were more usually made of polished mahogany lined internally with iron; but there has of late been a consensus of opinion in favour of metal only.

In the cheaper lanterns this metal body is usually made either of Russian iron or of sheet-iron tinned and j.a.panned, {71} there being little to choose either in price or quality between the two varieties, and in all but the very cheapest instruments the front is usually of bra.s.s.

In better lanterns the body is more often made of enamelled steel, the front as before being of bra.s.s; but bra.s.s, copper, or aluminium are also used occasionally for the body of the lantern.

[Ill.u.s.tration: FIG. 42.--Hughes' Short-Range Lantern.]

In deciding upon the type of body to be purchased the main considerations to be borne in mind are: (1) The type or types of illuminant to be used, a powerful arc lamp for example requiring a larger body than is necessary for a weaker radiant; (2) the size and position of the lens to be carried, a Petzval objective of say 3 inches diameter which has to be supported at the end of a long bra.s.s mount for long-range work obviously demanding a body of greater strength and rigidity than is required with a 6-inch focus lens of 2 inches diameter; (3) price.

Fig. 42 shows an extremely good lantern body for short-range work made by Messrs. Hughes, the ill.u.s.tration depicting the instrument complete with a 'Luna' methylated spirit lamp, though, of course, any other illuminant suitable for a small lantern could be used instead. {72}

This lantern ill.u.s.trates well one point that has already been emphasised as important, viz. the ventilation of the condenser. It will be noticed that this is placed _outside_ the body of the instrument instead of inside as is usual with larger bodies, and that wide slots are cut in the condenser mount to allow free escape of steam.

Other points of this excellent design are the screw adjustment to the slide stage (facilitating the use of special slides, such, for example, as those ill.u.s.trating the movements of the planetary bodies which sometimes involve the use of extra thick frames) and a simple but efficient tilting arrangement to the base.

[Ill.u.s.tration: FIG. 43.--Long-Range Lantern.]

Such a lantern is hardly suitable for a powerful arc lamp or limelight jet, or for heavy long-range lenses, but is a very good typical instrument for use in moderate-sized halls, and a lantern of this general type is usually found in lantern catalogues, though, of course, the exact designs vary according to the ideas of the manufacturer. Of lanterns for long-range work a good example is perhaps Messrs. Newton & Co.'s 'Intermediate' pattern (Fig. 43).

This again is only typical of many others by the various makers, but the princ.i.p.al points are common to all. These are: (1) The large and well-ventilated body; (2) the long {73} baseboard; (3) the strong and ma.s.sive bra.s.s front necessary to carry the large long-range lenses; (4) the velvet curtain at the back to close in any stray light from a powerful arc lamp.

[Ill.u.s.tration: FIG. 44.--Connections for a Bi-unial Lantern.]

The two foregoing designs are perhaps sufficiently typical of lantern bodies in general to make further detailed description of individual designs unnecessary; but reference should be made to features which special requirements may render advisable.

Under this heading mention must be made of _Bi-unials_ or Double Lanterns, as used for the once famous 'Dissolving Views.'

A bi-unial lantern consists essentially of two different instruments, each complete with its limelight jet or other illuminant--front, condensers, objective, &c., usually mounted on one body--and with some arrangement for 'dissolving' or turning the light in each lantern gradually on and off.

Fig. 44 shows the back view of such a lantern with two limelight jets and dissolving tap, this piece of mechanism (shown below in the ill.u.s.tration) being so arranged that when the lever is horizontal _both_ lanterns are on full, but moving the lever either way cuts off the gas supply to one lantern. In the case of limelight the tap should always operate by cutting off the oxygen supply in advance of the coal gas (in order to avoid a 'snap'), and the latter should never be cut off entirely, but a small bead of flame left to keep the jet alight, until the lantern is required for the next slide. {74}

This is usually arranged for by means of a bye-pa.s.s, and a bye-pa.s.s is sometimes provided on the oxygen side as well, but is usually discarded in practice.

A bi-unial lantern can be worked in the same way with acetylene gas, but with the electric arc it is impossible to turn the light on and off gradually, and in practice dissolving must be done by keeping both lanterns fully alight, and using a dissolving shutter, that is a movable shutter that covers each objective alternately. The same arrangement must be used with other illuminants, such as oil, only in this case the lanterns must be mounted side by side, on account of the tall chimneys. With oil lamps the arrangement answers fairly well, the dissolving fan, as it is termed, being made with serrated edges which give the _gradual_ obliteration required; but with the electric arc the extremely sharp definition becomes a serious difficulty, and a good dissolver for this illuminant has never yet been found, though, in view of the fact that dissolving views are more or less a thing of the past, the matter cannot be regarded as important.

The advantages claimed for a double lantern are two: first, a 'Dissolving'

effect by which one picture fades gradually into the next, and which is supposed to be more pleasing than the movement of a carrier; and second, 'Dissolving Effects' can be shown, such as exhibiting a landscape by day and changing it into a moonlight scene, or bringing on the appearance of a snowstorm, which can easily be done by means of a roller slide, with minute perforations shown in motion by the second lantern while the landscape remains on the screen from the first. In the days when dissolving views were all the vogue, a third or even a fourth lantern has been added for more complicated effects, and at the famous Polytechnic demonstrations of years ago, I believe that as many as six were sometimes employed.

In these days of the cinematograph it is doubtful how far interest in such effects could be revived, and a lantern has {75} gradually come to be looked on more as an instrument for showing ill.u.s.trations as required by the lecturer rather than as a pleasing exhibition in itself, and as dissolving views have lost their attraction, the double or triple lantern has been relegated to the limbo of antiquity.

[Ill.u.s.tration: FIG. 45.--Beard's Circulating Water Tank.]

Among other 'special' lanterns should be mentioned models made with water-cooled stages, for use with very delicate slides. This elaboration is not necessary with ordinary slides and illuminants of moderate power, but where very delicate slides, such as specimens of natural colour photography, have to be shown, it is an advisable precaution to pa.s.s the beam of light first through a tank of water in order to absorb the heat.

Lanterns intended for this work are usually constructed with a kind of double stage, a gla.s.s trough of water fitting into the rear aperture and the slide-carrier into the front one. Such an arrangement answers quite well for most purposes, but for extreme cases lanterns are equipped with a trough connected to a large outside tank and complete circulatory system, after the manner of the cooling tank of a gas engine.

Such a lantern, constructed by Messrs. Beard, is ill.u.s.trated in Fig. 45, and it will be seen that in this instrument the water trough is placed between the lenses of the condenser. {76} This is a very good position, as the beam of light at this point is, or should be, parallel, whereas between the condenser and the slide it is convergent, and therefore a condenser of a larger diameter than the slide must be employed in the latter case if the trough is of considerable width.