The Spectrometer is a device seen in the expansion "Return to Dragon Island" of the game School of Dragons.
Prior to the events in "Return to Dragon Island", Hiccup came with the idea of making a device that would allow him to see what chemical element are in the composition of certain substances. In order to complete his invention, he asked Johann for a prism.
When a toxic fog begins surrounding the School, Hiccup decides to finish the Spectrometer and use it find out what is the fog made of. He sends the player to Johann to collect the prism, the final piece for the invention. Then, Hiccup tells the player to go to Heather, who explains how the device works. She then tells the player to enter the lab and test how a few elements are seen through the spectrometer, including oxygen, hydrogen, sulfur and copper.
Later, after collecting a sample of the fog, the player uses the spectrometer on it and discovers that it is sulfur oxyde, which is toxic to humans. Heather then wants to test what the dragon fires are made of. She first does it with Toothless, revealing that his blast is made of acetylene and oxygen. She then goes to the Wilderness, bringing Stormfly, Meatlug and a Helpful Scuttleclaw. Using the spectrometer, Heather finds out that Deadly Nadder's fire is made of magnesium, Scuttleclaws fire is made out of copper and Gronckle's fire is made out of various elements, depending on the rocks it eats.