His early writings had been seen by Fontenelle and after his disappointment at failing to get to China, he persuaded Castel to go to Paris. Indeed late in 1720 Castel did go to Paris and taught physics and mathematics at the Jesuit school in the rue Saint-Jaques which was later to become the Lycée Louis-le-Grand. In particular he taught infinitesimal calculus and mechanics at the Lycée. Castel was never to leave Paris again except for a short visit to the South of France near the end of his life.
Immediately on arriving in Paris, Castel was appointed as an associate editor of the Journal de Trévoux, remaining on the editorial board of this monthly publication until 1745. Castel was a strong opponent of Newton's views on science and he made these views clear in his two volume work Traité de physique sur la pesanteur universelle des corps Ⓣ (1724). On nationalistic grounds he supported the views of Descartes and his opposition delayed the acceptance of Newton's theories in France. Castel believed that science should be based on logical thought not on experiment. Newton, he said:-
... reduced man to using only his eyes.Castel's physics was based on reason, not observation. He also opposed Newton on religious grounds, believing Newtonian theory to be materialistic. He made this clear in an early article written in the Journal de Trévoux in 1721 in which he stated that Newton had been influenced by Democritus in substituting the void for divine intelligence. He gave his own alternative system to replace Newton's system but it is of little importance :-
It was an attempt to harmonise philosophy, scientific curiosity, and religious dogma by methods of rationalism.There was another reason why Castel opposed Newton. Science, he believed, had to be accessible to everyone and so both higher mathematics and costly experimentations had to be excluded from scientific methods. He wrote in the Journal de Trévoux that:-
... the experiments capable of perfecting physics, ought to be easy to make and to repeat at any time, and almost by everyone.He published Vrai système de physique de M Isaac Newton in 1743 in which he claimed (with some degree of exaggeration!) that:-
... in order to make these experiments on the refraction of light correctly one must be a millionaire.Castel's ideas on the accessibility of science were also held by others such as Diderot, a friend of Castel. Diderot expressed similar views to Castel in Lettre sur les aveugles Ⓣ (1749) and De l'interprétation de la naturei Ⓣ (1751).
Castel's system to replace the theories of Newton did not bring him fame. However he did achieve this from a rather unusual source. In the November 1725 issue of Mercure de France he set out his ideas for an instrument, the clavecin oculair, which made colours and musical tones correspond. Two articles in the Journal de Trévoux in 1735, namely Nouvelles expériences d'optique et d'acoustique Ⓣ and L'optique des couleurs fondée sur les simples observations Ⓣ, took the idea further describing an instrument to accomplish the colour-tone correspondence, namely the ocular harpsichord :-
By 1742 the fame of Castel and of his invention had reached as far as St Petersburg and had been brought to the attention of the empress. the instrument was completed in July 1754, and on 21 December of the same year Castel gave a private demonstration of it before fifty guests. The spectators were enthusiastic and applauded several times. Colour organs appeared in the early 20th century in England and the United States.Castel was elected to the Royal Society of London in 1730. He was also elected to the Bordeaux Academy (1746), the Academy of Rouen (1748) and the Academy of Lyon (1748).
Article by: J J O'Connor and E F Robertson