I am extremely grateful to Richard in California, who generously sent me the above book as a gift, along with a very interesting collection of aura photographs. The book was first published in 1941 as an English translation of the 1938 second French edition. Considering this was written over 70 years ago I am absolutely astounded by the accuracy with which it explains in detail how:
Under the action of the electric field of the Earth, any body emits:
1. On all surfaces and both in the direction of the zenith and in the opposite direction, a thick bundle of vertical rays.
2. On a special line of that surface, two thin sheets of rays with the respective inclination of 45° and 135° on the vertical.

The author goes on to say:
We shall conclude that, from the dowsing point of view, it is as if, from ultra-violet to medium green, light waves were accompanied by ES [Electrical Specific] waves and of MS [Magnetic Specific] waves, from infra red to medium green with alternation of polarities, at the passage of each of the seven neutral colours, and that the same carriers, with similar alternations, of polarities, extend to the ultra-violet and infra-red parts of the spectrum.

The book, in effect, not only explains how dowsing works but also aura photography and many long-range locators, which weren’t even dreamt of at the time of writing. Reading this book has been an enlightening experience for me and I am sure I will be referring to it often. A reprinted version of the book is available at $19.70 from: http://www.healthresearchbooks.com/pages/book_detail.php?pid=84