[5] Dr. Harriette Chick and her co-workers of the Vienna University Child Clinic discovered, first, that the action of cod liver oil on the bone-lesions of rickets has an exact parallel in that of the ultra-violet rays of sunlight, or of the rays from a mercury-vapor quartz lamp; and, second, that the oil and the rays were effective substitutes the one for the other. See my _Man’s Debt to the Sun_, Little Blue Book No. 808, page 49. [6] The only creature that has porphyrin as part of its normal body-covering is a tropical bird called the touraco, parts of whose feathers are dyed a brilliant red by a porphyrin-copper compound known as turacin. This pigment is remarkable also because it seems to be the only normal occurrence of copper as a coloring compound in feathers or skin. Turacin is soluble in weak alkali, so that when it rains and the bird comes into contact with such alkaline solutes as frequently occur in nature, the turacin bleaches out! Although porphyrin is rare as a normal coloring in adult animals, it is the commonest pigment found in the shells of birds’ eggs. Almost all eggs, from the hen’s brown to the robin’s blue, contain it. [7] The length of the very short X-rays was accurately determined by a new method developed by Compton and Doan in 1925, and was found to be about three billionths of an inch. Transcriber’s Notes pg 19 Changed: In March, 1923, Dr. I. Seth Hirsh to: In March, 1923, Dr. I. Seth Hirsch pg 42 Changed: unable to refer to their soure to: unable to refer to their source