Peter Salcher (1848 – 1928), professor at the Rijeka Naval Academy, was a close associate of Ernst Mach and succeeded in what the famous physicist could not achieve - to make a picture of the invisible. Mach wanted to provide experimental evidence of his hypothesis about the existence of a shock wave around objects moving at speeds greater than the speed of sound. Therefore, he asked Salcher to try gaining such evidence in his laboratory in Rijeka. No sooner said than done. 

Peter Salcher, author of the first ultra-fast photo of a shock wave
around flying gun bullet

In 1886, Salcher and his associate Sandoro Riegler took a series of ultra-fast photographs of the acoustic phenomena that arise around a flying gun bullet, thus proving the existence of the shock wave, today also known as the sound barrier.

The Salcher photo of the shock wave around flying gun bullet. Source.

His work was a scientific sensation. Could the unit for the speed of sound, instead of Mach, have been Salcher? We'll leave that question unanswered and just remember that Salcher during his experiment actually took the first ever photographs of – a flying bullet. Source of the text www.kvarner.hr . 

One of the students of  Peter Salcher was Ludwig von Trapp.