Optical Projection.

by Lewis Wright and Russell S. Wright.

PREFACE.

The first edition of this work was written by my father, the late Mr. Lewis Wright, and was published in 1890.

The reception that it received testified to the fact that it met a long-felt want, and successive editions were published in 1895, 1901, and 1906.

My father, unfortunately, met his death in a railway accident in 1905, and the corrections and additions to the last edition, which had been to a certain extent prepared by him, were completed and written by myself, and the work as published then was again reprinted in 1911.

As the original text is now thirty years old, it has seemed better entirely to re-write the whole book rather than make fresh revisions, the more so as the last ten years have seen great advances in the science of Lantern Projection, and especially in the developments of Acetylene and Electric Lighting.

It has also seemed best at the present juncture to issue the book in two parts, the first dealing with the Projection of Lantern Slides only, and the second with the Demonstration of Opaque and Microscopic Objects, Scientific Phenomena and accessory apparatus, including Cinematograph Projection.

It must of necessity be many months before this second volume can be produced, for the simple reason that Optical {vi} Instrument Makers have as yet hardly had time to turn round after the war and produce their new models, and therefore any such book written now could do little more than describe apparatus that was on the market prior to 1914.

The present work, therefore, deals solely with the exhibition of Lantern Slides in the Optical Lantern, and as such I trust will be found of value to Schoolmasters, Social Workers, Lecturers, and, in fact, to all who use the lantern as a means of ill.u.s.tration.

RUSSELL S. WRIGHT.

_January 1920._

OPTICAL PROJECTION

A TREATISE ON THE USE OF THE OPTICAL LANTERN

CHAPTER I

INTRODUCTORY

Lantern Projection, as commonly understood, may be broadly subdivided into two branches: (A) The Projection of Lantern Slides, and (B) The Projection of Scientific Phenomena, Opaque Objects, Microscopic Specimens, &c., usually referred to broadly under the heading of 'Scientific Demonstration.'

To these two cla.s.ses may perhaps now be added a third, viz. The Projection of So-called Living Pictures, or, in other words, the Cinematograph. In the earlier editions of this work both A and B were dealt with in the same volume, but, as there are thousands who require to use a lantern for the demonstration of lantern slides only, and who have no interest or concern with Science Projection, it has seemed to the writer that the work might, with advantage, be divided into two portions, Vol. I. dealing with slides only, and Vol. II. with the various adaptations of the science lantern.

This present book therefore only deals with the exhibition of lantern slides, and as such it will, I trust, be found to be of real a.s.sistance to the ordinary user of the optical lantern, including clergymen, schoolmasters, army and cadet officers, and others {2} who require advice and instruction in the purchase or use of a lantern.

The essential parts of a lantern are: (_a_) A _slide-holder_ or _carrier_ to hold the slide; (_b_) a _lens_ to 'focus' it on the screen; (_c_) a _condenser_ to converge the light upon slide and lens; (_d_) a source of light or _radiant_ to provide the necessary illumination; and (_e_) a _body_ or framework to hold the whole together. All possible variations in choice of a suitable lantern relate to one or another of the above parts, and will be treated of in turn; but, fortunately, we have this all-important simplification that every ordinary English lantern slide is the same _standard size_, viz. 3 inches square. Some Continental and American slides differ in one dimension from the above, but not enough to cause any serious difficulty, and the convenient English standard is being gradually adopted throughout the world.

The varieties of slide-holders or carriers are therefore comparatively few and are chiefly concerned with the question of rapidly and easily changing the slides. The choice of a focussing lens or objective is mainly a matter of the size of picture required, and the most convenient distance from the screen for the lantern to be placed. Variations in condensers, which are comparatively small, are usually only a matter of conforming these with the size or type of objective to be used, and should be left to the manufacturer's judgment. The question of a suitable radiant is partly a matter of the amount of illumination required, and partly that of the practical possibilities; for example, if electric current is available some form of electric light is usually the most convenient, as well as the least expensive, but where this is not the case, paraffin-oil, methylated spirit, incandescent gas, acetylene, limelight, &c., are alternatives which all have their uses and must be considered on their own merits.

Sometimes, as for example in the case of a travelling lecturer, a lantern is required fitted with a range of lenses for {3} halls of different size, and also with a variety of illuminants, and this in most lanterns can be easily provided for.

The body is usually a matter of taste and price only, and may range from a simple but efficient sh.e.l.l of Russian iron to an elaborate mahogany instrument with a bra.s.s front, screw tilting arrangements and other adornments; but of late years there has been a wholesome reaction against unnecessary finish, and a simple metal body of some description is now chiefly the order of the day. In the foregoing remarks the various parts of a lantern have been mentioned in what I should consider the correct order, starting from the slide and slide-holder, and so to speak building up the rest of the instrument round these items; but I now propose somewhat to vary the procedure and for convenience deal in detail first with the Radiant, or _Illuminant_.

CHAPTER II

THE ILLUMINANT

The first necessity for lantern projection is a strong light, and this can be obtained from a variety of sources, the princ.i.p.al means in common use being approximately in order of excellence as follows: paraffin-oil, incandescent spirit, incandescent gas, acetylene, acetylene air blast, oxyhydrogen (limelight), oxyether, and electric light in its various forms.

The ideal characteristics to be sought for are (1) great intrinsic brilliancy; (2) minimum _size_ of luminous spot; (3) freedom from flicker; (4) freedom from smell; (5) absence of any preponderating colour; (6) cheapness; and (7) convenience. There is no question whatever as to which of the available sources of light most perfectly combines all the above if it is available, viz. the electric arc. If a current supply is in {4} the building, this form of lighting easily excels all others, except possibly in the matter of flicker, and even in this respect there is very little fault to be found with it.

From all other points of view it is wellnigh perfect, inasmuch as it provides an extremely concentrated and intensely luminous spot, of almost perfect whiteness (if anything slightly bluish), no smell, comparatively little heat, convenient and inexpensive. So great is the advantage of the electric arc that attempts have been made to use it from acc.u.mulators in places where a current supply is not available, but this cannot be seriously recommended, except in special cases. Where an electric supply is, however, available there can be no real choice, whether the lantern is required for use in a large hall or a small cla.s.s-room. The advantages of using the arc are so great that no other method need be seriously considered.

The one real objection that I have heard urged against it is due, curiously enough, to its very perfection, and that is, that it lends itself to such exceedingly sharp definition that any slight imperfection in the slide is too faithfully reproduced on the screen, for which reason it is sometimes recommended that the operator shall work with the objective the least fraction out of focus; but this is a matter for individual taste and judgment.

If, however, there is no possibility of using the electric current, one of the other sources of illumination must perforce be adopted, and for a _large_ hall this can only be limelight in one of its many forms, viz.

oxyhydrogen, oxyether, oxyacetylene, &c. As regards results on the screen, this light compares well even against the electric arc, but it involves the expense and trouble of compressed gas cylinders, or the infinitely worse recourse to the now obsolete method of filling gas-bags.

Limelight is therefore now but little used in this country, as the majority of large halls are equipped with the electric {5} current, and for smaller buildings it is deemed unnecessary and too expensive.

ACETYLENE is undoubtedly the illuminant most in favour next to electric light, as the light is brilliant enough to illuminate a picture 12 feet in diameter at a distance up to, say, 30 feet from the screen, and this suffices in a large majority of cases, and acetylene is comparatively cheap, and reasonably simple to work.

INCANDESCENT-GAS is often employed for small cla.s.s-rooms and is fairly effective for a picture not exceeding 9 or 10 feet in diameter, and the same can be said of the same type of burner heated by methylated spirit.

PARAFFIN-OIL is the poorest of all present-day forms of lantern illuminants. The flame is large, impairing the definition, yellow in colour, uneven in illumination, liable to smoke and smell, and barely equal to incandescent gas in illuminating power.

It is therefore going gradually out of use in this country, but in out-of-the-way places, especially abroad, it is sometimes the only practicable light, and is therefore still employed from the best of all reasons, necessity.

It is not the intention of the author to give precise working instruction for all and every variety of the above illuminants as manufactured by different firms. For such the reader must be referred to the directions usually issued by the makers themselves, but a general description of the various types offered for choice will not be out of place, and it will be more convenient to begin with the poorest, viz. paraffin-oil, and finish with the most perfect, the electric arc.

{6}

CHAPTER III

PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS

[Ill.u.s.tration: FIG. 1.--Oil Lamp.]

There are several varieties of oil lamps on the market, but in practically every case they take the same general form, a metal reservoir sliding in grooves in the lantern body and holding approximately a pint of oil with (usually) four wicks _nearly_ parallel, but slightly converging from rear to front, these enclosed in a flame chamber of Russian iron, with _loose_ well-annealed ends of sheet gla.s.s and an adjustable reflector at the back, or sometimes the reflector itself forms the rear end of the flame chamber.

The chimney must be tall and is now usually made adjustable, though I have never been able to trace any real advantage from this complication {7} (Fig. 1). The whole secret of obtaining the best results from these lamps may be summed up--_good oil and perfect cleanliness_; and it is wonderful what can be done when these points are properly attended to.

Care should be taken in tr.i.m.m.i.n.g the wicks to see that no charred parts fall down between the wick holders, but it makes little difference whether the tr.i.m.m.i.n.g is done with scissors or by rubbing with the finger. Special lamp scissors are sold by all makers with a large flat on one side to catch the portions cut off.

These lamps should be well rubbed over the last thing before use, as paraffin-oil is apt to 'creep,' and the operator does not want to be told that his apparatus is suggestive of a fried fish shop. In working with these lamps it is difficult to avoid a dark streak down the centre of the sheet, representing the s.p.a.ce between the two centre wicks; to a certain extent this can be obviated by adjusting the reflector, and in any case is not very obvious when the slide is in place. Lamps constructed with either three or five wicks are better in this respect, but the former are usually considered to be too poor in illuminating power, and the latter are apt to crack the sheet-gla.s.s ends by excessive heat.

INCANDESCENT GAS.--Incandescent gas burners do not need much description, as they are practically similar to those in general use for house lighting.

They may be either of the erect or inverted forms, the latter being preferable owing to the light being more concentrated, and a reflector is provided to increase the illumination (Fig. 2).