Letter from Jean-Baptiste Biot to Thomas Young Delighted to have read Young's critique of his early works on optics. Hopes he will read his later experimental work as it furthers his findings in particular on polarisation. Includes a demonstration on the polarisation of light through crystals to discuss how light splits to support the theory that light moves as molecules rather than waves. Has also presented a new instrument to support his optical experiments at the Institut [de France]. As a postscriptum reassures Young that there was no space between the two slides in his experimentation and includes a small figure in the margin to show who the water was placed between the glass slides. nd