Simple inside inspecting device

Dear all,
it happened recently that a couple of members was not sure if the glass bottle that they were showing was a blown glass one or if it was carved from a solid block of glass.
This is an old problem. In my opinion, bottles carved from solid block of glass are rarer than what commonly believed.
Anyway, there is a simple way to determine that.
Consider this: suppose that you could turn out the inside of the bottle. If that was possible, you surely will be able to determine if that surface has been carved and polished or if it is the natural surface of a blown glass. That is because, for how much fine the polishing could be, the natural surface of a glass has a typical looking, probably because of the surface strength of the glass.
This evaluation is not possible if you look through the mouth hole of a glass bottle, because the inner lighting is becoming through the transparency of the glass.
In fact, one can see and judge the surface of a glass only when it is reflecting the light, not if the light is becoming through it, thanks to its transparency.
Hence, we need to light the inside of the bottle FROM THE INSIDE, not through the glass, from outside.
I know that there are commercial devices that allow you to light the bottle from the inside. But these has a flexible neck that is too thick, leaving almost no free space to look through the mouth of the bottle.
We need something that will occupy a very small quantity of mouth’ space when it is inserted.
Here are the step-by-step instructions on how to do that. Apologize if I am saying things that are obvious to many, but not everybody has technical basics.
Well, all what is necessary is shown in the first picture. We need, as seen from top to down, left to right:
-   A soldering iron
-   Some soldering tin
-   A small power supply, of the lowest power that you can find, having a voltage selector tap of 3V dc. (3V = 3 Volt. Volt, not volts, as you Americans wrongly say 😊)
-   One feet of the smallest wire that you can find. You can either use a pair of thin wires, as seen in the picture, or a thin double wire cord. You could cut it from an old, disposed cell phone power supply, for example.
-   A mini LED of white color. Mini LEDs has a diameter of 3 mm, hence they are suitable to pass through any snuff bottle’s mouth.
Not everybody has a soldering iron. But everybody knows somebody that should have it, either for hobby or profession.
What we need to do is to connect the LED to the power supply. In doing that, we must keep in mind that the power supply is providing 3V dc, direct current, hence it has a polarity, and the LED too has a polarity. The polarity must be respected, because if not, the LED will not light. But no worry, as we will see.
The second image below is a drawing showing a LED in detail. You can see that the plastic case of the LED has an edge in relief, which makes its diameter larger than 3mm. This edge has a flat side, allowing to indicate the negative leg of the LED, when legs are cut out. You could file this edge off, no worry if this will cancel the indication of the polarity.
Well, first, we have to cut out the original plug of the power supply. Then shorten one of the two wires by cutting away about one inch.
Weld the two thin wires to the two wires of the power supply. The trick of shortening one of the two wires is for having the two connections in two different places along the cord, which will avoid the possibility of the two wires being in electrical contact (the so called short-circuit).
Now pair the ends of the two thin wires and remove a few millimeters of the insulation from them.
Then shorten the legs of the LED to a few millimeters.
At this point, keeping the two thin wires separated, plug the power supply into power socket and briefly put in contact the two wires to the two shortened legs of the LED. If the LED will turn on, then the polarity is correct. If not, reverse the two wires and you will see that the LED will light.
You must now remember which wire must be connected to which leg of the LED, and weld them accordingly. The welding procedure must be relatively fast, for avoiding over heating of the LED and melting of the plastic case. If you have a hobbyist friend, ask him to do that.
The third picture is showing the LED connected to the thin wires. Note that it is not necessary to insulate this connection, because the shortened legs are enough rigid for keeping them separated, and because of the very low voltage.
The final assembly is shown in picture 4.
Now it is just matter of introducing the LED through the mouth, looking to the inside with a lens, and moving the LED around by means of the wires.
I found this simple device very useful, for example for seeing if the core of a lacquer bottle is made of brass, copper or wood. I have found that a bottle that I thought was a kind of stone was indeed a Yixing bottle.
Kind regards
Giovanni