How do instant photographs work?

In order to answer the question of how exactly Polaroid cameras are able to take pictures and have them fully developed only moments later, some information on the structure of the film used in instant cameras is needed.

The Polaroid film itself consists of the same three light-sensitive layers used in conventional cameras, which are separated from each other by dye coupler layers. An image layer, along with a timing layer and an acid layer are placed on top of them. A black base layer forms the undermost layer of all of the other layers mentioned above. Probably the most important layer is the layer situated between the light-sensitive layer and the image layer, which contains a chemical called the re-agent. 
The re-agent is a mixture of opacifiers, which block light; alkali, necessary for neutralizing acids; and white pigments. The entire frame of the film is covered in a combination of these elements. The re-agent itself starts the chemical reaction taking place in Polaroid film.

The chemical processes start once the Polaroid camera pushes the film out, using rollers inside the camera. The re-agent is spread across the whole film, which starts a series of chemical reactions. The colors from the dye layers move up from the lower layers, the acid layer moves towards the top and the re-agent moves towards the lower layers. Another chemical reaction occurs once the re-agent and the acids meet. The alkali and the opacifier in the re-agent react with the acids to make the re-agent turn clear. Following this reaction the image starts to appear until the film is, at last, fully developed.

Group/Instructional: How to build a camera lucida

Rating the instructions of ‘How to make a camera lucida'

Though the instructions on “how to make a camera lucida” are not completely useless, there are several aspects of the instruction that could be improved. One of the aspects that proves that the instructions are insufficient is the fact that they rely heavily on the use of images. In fact, they rely on pictures to the point that the instructions become incomprehensible once they are removed from the text.

The text that accompanies the images does not give any precise instructions and is, for several reasons, confusing. One of those reasons is that it lacks a clear chronological order. This can confuse those people who have never seen or constructed a camera lucida before and therefore do not know about the logical order of the steps taken to build one.

Another problem is that no hints or warnings are mentioned, which could prevent accidents and ensure a successful construction of the camera lucida. Just as serious as the lack of hints and warnings, is the fact that it does not include a list of what materials should be used. Many readers might, for example, choose a piece of clear plastic unfit for the use in a camera lucida and therefore not achieve the expected outcome.

The fact that the title does not even hint at the fact that the text includes both, the instructions on how to build a camera lucida, but also how to use it, could lead to more misunderstandings. Equally, the text should be divided into separate parts for the construction of a camera lucida and its use for drawing.

If the text were to be adjusted in the points mentioned above, the instructions could easily be rendered comprehensible and useful to its readers.