APA (7th ed.) Citation

Planck, M., Clarke, H. T., & Silberstein, L. (2010). The Origin and Development of the Quantum Theory. Project Gutenberg.

Chicago Style (17th ed.) Citation

Planck, Max, H. T. Clarke, and L. Silberstein. The Origin and Development of the Quantum Theory. Project Gutenberg, 2010.

MLA (9th ed.) Citation

Planck, Max, et al. The Origin and Development of the Quantum Theory. Project Gutenberg, 2010.

Warning: These citations may not always be 100% accurate.