THE LANTERN MICROSCOPE.
Of late years it has become possible to take very beautiful photographs of microscopic objects, which, when used as lantern-slides, give a very fine effect. Numerous slides of this character may now be purchased, which cover a vast field of microscopic knowledge, and many will be content with what they can thus obtain. But there is another class of workers who, perhaps possessing a large number of microscopic slides, will be glad to utilize them for the lantern, so that they are no longer limited to the individual eye. For these the lantern microscope, which will screw in front of the lantern in place of the usual optical system, is a most useful instrument. Different makers advertise in their catalogues lantern microscopes which range in price from twenty-five shillings to as many guineas. And it recently came to my knowledge that a well-known London optician has undertaken the construction of a few of these instruments at the price of £150 each. The amateur lanternist will, therefore, have some difficulty in deciding what he ought to pay for such an instrument. And it is most difficult to advise him on that point. His best plan is, perhaps, to see for himself what the makers can offer him, and, if possible, to seek a trial of the instrument before he purchases it.
The lantern microscope is a modification of the old solar microscope—an instrument which could be screwed against a hole in the shutter of a darkened room—and which had a mirror outside which projected a beam of sunlight through it. The objective lenses were uncorrected, and small in aperture, so that the instrument was far from being a perfect one. But the intensity of the solar light covered many objectionable features, and it was possible to get some astonishing effects with the contrivance. But sunlight is such an uncertain commodity in our climate that, when the lime-light came into prominence, the old solar microscope was forgotten in favour of that which depended upon a more constant, if a far less intense, light source.
The great difficulty to overcome in the lantern microscope is to get the light sufficiently intense to well illuminate the objects. The aperture of a microscope objective, even of low power, is so small, that, when the pencil of light which it emits is spread over a screen, the light is so attenuated that the details of the object are difficult to make out. In the near future, when we are promised that electricity shall be laid on to our homes, as gas is now supplied to us, the difficulty will disappear, for the electric-arc light is many times more intense than the most powerful lime-jet.
Having obtained a lantern microscope, the next thing to consider is the class of objects which can best be shown with it. To help the reader in determining this point, I will now give a kind of sketch of a popular lecture to accompany an exhibition of the instrument.
Such a lecture might usefully commence with a brief description of the microscope: its history, and how, in its improved form, it has opened up a world of minute life, of which our forefathers could have had no conception. We might then put forward a few remarks as to the adaptation of the microscope to the oxyhydrogen lantern, and point out how by its aid a number of persons can at the same time see what, in an ordinary microscope, is visible to only one person. So much by way of introduction.
We might next enlarge upon the circumstance that the contents of the world have been classified into three general divisions—popularly known as the three kingdoms of Nature: the Mineral Kingdom, the Vegetable Kingdom, and the Animal Kingdom. Show how one is dependent upon the other, and the difficulty in many cases of drawing a line to separate the minute organisms of the Animal Kingdom from those of the Vegetable World. Numerous examples from the lower forms of life may be quoted to illustrate this. Now show and explain subjects from each of the three divisions, taking the minerals first. Define a mineral, and show how they crystallize in definite forms. Illustrate the process of crystallization by putting a drop of a saturated solution of sal-ammoniac on a slip of glass, and spreading it over with a brush. Such a glass placed in the microscope will, as it dries, become covered with branches of crystals, which seem to grow and strike out in various directions on the sheet in a truly marvellous manner. Sections of various minerals may next be shown, finishing with sections of coal. In pointing out the origin of coal from plants, we may easily drift into the next division of our subject—the Vegetable Kingdom.
Here we have indeed a wide field to illustrate. Commencing from the simple cell, as exhibited, for instance, in a bit of so-called green mould, and arriving gradually at sections of different stems of plants (which by the way form most beautiful objects on the screen), sections of different woods, parts of flowers, etc. From these it will be easy to step over the indistinct boundary-line which separates us from the Animal World. Here we can commence with the lowest organisms; next show the living inhabitants of our ponds and ditches. Arrive at the Insect World, and show the generally unfamiliar parts of familiar insects—bees, wasps, flies, etc. We can then go higher, and exhibit sections of bone, hair, etc.; finishing with the structure of the human body, so far as it can be indicated by means of the microscope. This is a brief sketch of the kind of entertainment the microscope is able to afford, which can be modified to suit the views of the exhibitor.