PART I
THROUGH A MICROSCOPE
By Samuel Wells
THROUGH A MICROSCOPE
I
An object one hundredth of an inch in diameter, or of which it would take one hundred placed side by side to make an inch, is about the smallest thing that can be easily seen by the unassisted eye. Take a piece of card and punch a little hole through it with the point of a small needle, hold it towards a lamp or a window, and you will see the light through it.
FIG. 1.
This hole will be about the size just mentioned, and you will find that you can see it best and most distinctly when you hold it at a certain distance from your eye; and this distance will not be far from ten inches, unless you are near-sighted. Now bring it towards your eye and you will find it becomes blurred and indistinct. You will see by this experiment that you cannot see things distinctly when held too close to your eye, or in other words, that you cannot bring your eye nearer to an object than eight or ten inches and see it well at the same time.
You could see things much smaller than one hundredth of an inch if you could get your eye close enough to them. How can that be done? By a microscope? yes, but what is that? This name comes from two Greek words that mean "to see small things;" and a microscope is an instrument by which your eye can get very close to what you want to see.
To understand this, take out one of your eyes and look at it with the other one. You see that it is a little round camera; most boys have seen a camera and some boys can make one. The simplest way to do that is to take a box, say a cigar box (empty, of course); pull off the cover and fasten in the place of it a piece of ground glass if you have one: if not a piece of white letter paper, oiled, will do; bore a hole in the middle of the bottom with a small gimlet and your camera is done. Point the bottom with the hole in it out of the window, and throw a piece of cloth over your head and over the box, as the photographers do, to shut out the side light, but mind and not cover up the hole; look at the ground glass (or oiled paper) and you will see things upside down. (Fig. 1.) But what has it to do with my eye? you say. Why, your eye is just like it, only round, as in fig. 2. And if you hold a doll or anything else about ten inches in front of the eye you have taken out and look at the inside of it (the eye, not the doll) just as you look at the ground glass of your box camera, you will see the doll upside down on the back of the eye.
But how, do you say, can I see things right side up when they are upside down in my eye? This is a very good conundrum and it will keep a long time, till you are about seventy years old and have spare time to sit down and think about it.
Now you see how your eye is a camera; the pupil is the hole and the back of the eye, called the retina, is the ground glass.
But you will find that the camera you have just made does not show things distinctly and beautifully as the photographer's camera does; how can they be distinct in the eye then?
Because in the photographer's camera, in the hole is a lens, which is a piece of glass, shaped like a sun glass; and so in your eye just behind the pupil is a lens, not made of glass, but still almost as transparent as if it were. In order to see what effect this lens has, take your box camera, make the hole larger and put a lens in it; one of your magic lantern lenses will do; and if the lens has the right focus you will see the images sharp and distinct on your ground glass. The focus probably will not be just right, so make a paper tube, into which fasten your lens and slide the tube in and out of the hole until you find the right focus.
When you have got that right so that you see a boy on the sidewalk upside down and see his teeth when he laughs, put some small object, the little doll will do, about three feet in front of your lens, and you will find the image of it is blurred and indistinct, and that you must pull your tube out to get the focus on the doll; or if you had another lens of just the right shape to hold in front of your camera, you would with that get the focus on the doll.
FIG. 2.
Thus you can see how it is with your eye, and why you cannot see things distinctly held close to it. The lens in the eye can change its shape a little, so that it will focus objects a mile off, or ten inches off, but it cannot be pushed in and out like the tube in your camera. You can do this, however, if you take another lens and hold it outside your eye and let the light go through that first before it comes to the lens in your eye, and in this way you can get a focus in your retina, and the outside lens thus forms a part of that optical instrument called your eye. Does your grandma know that her spectacles are a part of the cameras that she calls her eyes?
How is it that a lens bends (refracts is the big word for it) the rays of light? You will learn by and by. You can see that it does so by a few experiments with your sun glass or any such lens. Hold it between the sun and a piece of white paper until the white spot in the centre is as small as you can make it. You will see that the rest of the lens casts a shadow although it is all glass; this is because the rays of sunlight that fall on the lens are all bent towards the centre, and so you have a small white spot on which is concentrated the light and the heat, and before you have found out how it is all done, your paper takes fire and the experiment ends in smoke.
Take another piece of paper, and when the white spot is at its smallest, measure the distance between the lens and the paper, and you will have the focal distance of the lens.
You have now found out how to get your eye close to an object and see something that is very small; this is usually called magnifying it, because it seems to make it look large. Suppose you have a lens that will let you see a flea through it held just one inch from it, this lens is now an addition to your eye, as we measure from the lens. If you had another flea held ten inches off, so big that it would just be hidden by the little flea, the one farthest off would be ten times as large as the near one. (Fig. 3.) In this case it is said that the lens having a focal length of one inch magnifies ten times, or has a power of ten.
FIG. 3.
The shortest usual distance of objects seen distinctly being taken as ten inches, microscopists have agreed to consider that as the standard of measurement, and objects seen through a lens are considered magnified to the size they would have if projected ten inches off, like our little flea.
