The most powerful of all is the true oxy-hydrogen jet. Here we have both gases (the oxygen and hydrogen) under a much higher pressure. The jet is so constructed that the two gases mix in the chamber of a jet, and pass through the jet into the lime cylinder. This jet is equal to 400-425 candles. The light is most intense, and with it views 35 ft. diam. may be shown. Its great drawback is that it is only safe in the hands of an experienced operator. It is a form requiring careful manipulation in every respect, but if certain conditions are complied with, it can be used without danger.

Highley describes an oxy- spirit lamp which seems perfect in all its arrangements. Pumphrey's vaporiser, when you cannot get coal gas, is an inexpensive apparatus, and can be used with any lantern, the spirit being volatilised, and burned without a wick at the mouth of an ordinary safety jet. There are, however, two or three points to be observed in order to ensure success. Much depends upon the construction of the burner. If you use an oxygen nozzle with a very small bore, and force the gas through it by strong pressure - say 56 lb. or more on the bag - there will be a dark nucleus in the centre of the incandescent lime spot, supposing the cylinder of lime to be somewhat near to the orifice of the jet. And not only so, but on looking into the anterior glass of the lens from the front, you will see the same dark centre, with a ring of light surrounding it, something like an annular eclipse of the sun. The explanation is that the two gases are travelling at so different a rate at the time of their emergence from the jet, that they do not mix properly, and hence there is an excess of oxygen in the middle of the flame, producing a cooling effect.

Some say that the greater the pressure on the gas bag in the oxy-calcium process, the better the light; but in my own practice I find quite the reverse, and have obtained the best results by using a low pressure not exceeding 28 lb. The oxygen then travels slowly, and mixes more thoroughly with the spirit vapour before the flame touches the lime. Quantity of oxygen, however, is important, and hence I enlarge the bore to 1/20 In. to compensate for the diminished pressure. A single trial will show the advantage of this enlargement of the bore, the light being better and more steady, whilst the lime requires to be turned seldom, or not at all.

Look at the matter from a common-sense point of view. In this oxy-calcium process you have spirit of benzoline vapour escaping slowly through a large orifice of 1/4 in. diameter, and you send a small and rapid stream of oxygen into the centre of it. Unless, therefore, the lime cylinder be at some distance, the two gases cannot mix. When, however, you use a larger stream, and one whose rate of travelling corresponds more nearly with that of the spirit, the admixture is perfect, and complete combustion is the result. At all events, whatever the theory may be, there is no doubt that in practice the latter method gives the best result.

The 1/20 in. oxygen orifice may be measured by an amateur sufficiently nearly by stamping it on a sheet of paper, and making five impressions, side by side, which ought then to measure 1/4 in.; or by picking out a stocking needle which exactly fits into the bore, and pricking a few holes on paper close together.

A beginner will, perhaps, find a difficulty in centring the jet in the lantern when it is heated by the small lamps employed with Pumphrey's vaporiser. To avoid this, cut a circle of paper the size of the condenser, with a small hole in the middle, and stick it on the back glass with a wafer. You will then see exactly the height at which the jet should be fixed to bring the lime spot on a line with the optical axis, and it will remain only to cut a half cork, or a piece of soft wood, and tie it, underneath, on to the supporting pillar, so that for the future the jet may be dropped on to it, and screwed up securely. I work the oxy-calcium process with the lime cylinder rather near to the jet, although not quite so near as in the oxy-hydrogen process. In all comparative experiments, the distance from the jet should be noted, as it has an influence upon the result. The soft limes are preferable to the hard, when low pressure is to be employed. As to construction see Figs. 60-70. If we suppose, side by side, two mahogany body lanterns, and if we place one of them on the top of the other, making the necessary arrangements as to carrying the trays, etc., we have the first principles of the bi-unial lantern.

In doing this we find an obstacle arises, the disc of the upper lantern does not coincide or register with that from the lower one. Hence we are compelled to either tilt the upper lantern downwards, or in some way to obviate this. If we, therefore, allow the stage carrying the condenser, slide holder, telescope front, and focus lens of the lower lantern to remain as it is, but fit the upper stage to the mahogany body with a hinge, and arrange for its adjustment, which can be done by having a spring to press it outwards and two milled-head screws to draw it back, we then are able to cause both discs to coincide on the screen. This plan was the one adopted by Highley. Other makers adopt a preferable arrangement, and make both stages movable. Assuming that, instead of two lanterns, one on the other, we make a body specially adapted, dimensions as follows would be .suitable: -

Fig. 60 shows the wood body; height 18 in., inside measurement 7 in. by 9 in., bottom projecting 3 1/2 in. The bottom, Fig. 61, 13 1/2 in. by 94 in. It is easy to make a lantern, which, for appearance, looks compact and portable; but nothing smaller than the sizes named will give an instrument that will work well with good ventilation. It is a somewhat costly matter if, in reducing space, we find some evening that the woodwork of the lantern is on fire, and that one or both condensers are damaged. Inside the body should be a lining of either sheet iron or tin plate. Figs. 62 to 65 show various views of this. It should be kept off the mahogany, so that an air space of 1/2-5/8 in. is left all round. This, when the lantern is in use, allows a current of air to pass up, and so prevents undue heating of the wood. Fig. 62 shows the side measuring 18 3/8 in. by 9 in., with holes 6 in. square. Fig. 63 is the back, 6 in. wide. A good way of holding up the lining is to screw it to wood blocks (the thickness of the air space), which, if placed in the angles, will not interfere with the air currents.