PRINCETON, N.J., Jan. 23 James Hillier, designer of the first practical electron microscope, has died in Princeton, N.J., at the age of 91.

The Canadian-born physicist died of a stroke Jan. 15, the Los Angeles Times reported.

The electron microscope invented by German engineers Ernst Ruska and Reinhold Rudenberg in 1931 had significant problems that made it impossible to produce accurate images at high magnification, the newspaper said.

Hillier and fellow University of Toronto graduate student Albert Prebus began working together in 1937, adapting the work of the German scientists into a prototype for the first working electron microscope.

The newspaper said Hillier spent another year perfecting his microscope before taking it to Radio Corp. of America in Camden, N.J., where he joined the research staff.

Within two years, he sold 50 instruments at $10,000 apiece and developed 50 pioneers in 50 different fields, the LA Times said.