Having described the harmony of the physical world at atomic and astronomical levels, he goes on to consider Nada Brahma, an Indian Yoga technique of 'listening to the internal sound current'. The technique consists essentially of blocking ones ears and listening to the 'sound currents' which do actually come when one performs the technique (I've tried it). By meditating on these sound currents, one is taken into deeper levels of concentration. This 'inner music' is related to the concept of the music of the spheres, and this notion is investigated historically.
Overall, the book is an excellent read, and I just regret I lent my copy to someone and never got it back. Maybe I'll have to get the next edition from Amazon.com, as it seems to be on their website. If you're a musician and want something spiritual, this meditation might be for you!