Turn the big wheel at the back of the installation slowly while you look into the glass container.
Look at the bulb and the test tubes. Why must the glassware be submerged and filled with oil to appear invisible?
Also try
Look at the text behind the lens. Is there any difference when the lens is above or submerged into the oil?
Fun facts
The speed of light in air is approximately 300 000 km/s, but only approx. 200 000 km/s in glass. When the light hits the glass, most of it will be refracted but still travel through the glass. However t some will be reflected, and it is the reflected light that enables us to see the glass.
Light is reflected upon surfaces where the speed of light differs from its surroundings. Since there is no difference between the speed of light in rapeseed oil and glass, the light will neither be refracted nor reflected and the glass will appear invisible. This is also the reason way the lens stops working when it is submerged in the oil.