1911, Max Reinganum showed that the passage of an 𝛂-particle at glancing incidence through a photographic emulsion produced, when the emulsion was developed, a row of silver halide grains outlining the trajectory of the 𝛂-particle; the first recorded observation of an extended particle track in an emulsion. The next steps would naturally have been to apply this technique to the detection and research of other particle types, including the Cosmic Rays newly discovered by Victor Hess in 1912. However, progress was halted by the onset of World War I in 1914. The outstanding issue of improving the particle detection performance of standard photographic emulsions, in order to detect other types of particle - protons, for example, produce about one quarter of the ionisation caused by an 𝛂-particle - was taken up again by various physical research laboratories in the 1920s. In particular Marietta Blau, working at the Institute for Radium Research, Vienna in Austria, began in 1923 to investigate alternative types of photographic emulsion plates for detection of protons, known as “H-rays” at that time. She used a radioactive source of 𝛂-particles to irradiate paraffin wax, which has a high content of hydrogen. An 𝛂-particle may collide with a hydrogen nucleus (proton), knocking that proton out of the wax and into the photographic emulsion, where it produces a visible track of silver halide grains. After many trials, using different plates and careful shielding of the emulsion from unwanted radiation, she succeeded in making the first ever observation of proton tracks in a nuclear emulsion. By an ingenious example of lateral thinking, she applied a similar method to make the first ever ‘observation’ of a neutron in nuclear emulsion. Being electrically neutral the neutron cannot, of course, be directly detected in a photographic emulsion, but if it strikes a proton in the emulsion, that recoiling proton can be detected. She used this method to determine the energy spectrum of neutrons resulting from specific nuclear reaction processes. She developed a method to determine proton energies by measuring the exposed grain density along their tracks (fast minimum ionising particles interact with fewer grains than slow particles). To record the long tracks of fast protons more accurately, she enlisted British film manufacturer Ilford (now Ilford Photo) to thicken the emulsion on its commercial plates, and she experimented with other emulsion parameters — grain size, latent image retention, development conditions — to improve the visibility