In anatomy it is common to experiment on fresh pig’s eyes in order to make comparisons with human eyes. The above named Frenchmen submitted the former to X-rays. The eyes were but slightly permeable thereto. [Illustration: THE PHYSICAL INSTITUTE, UNIVERSITY OF WÜRZBURG, WHERE PROF. ROENTGEN HAS HIS RESIDENCE, DELIVERS HIS LECTURES, AND PERFORMS HIS EXPERIMENTS. From photograph by G. Glock, Würzburg. (Not referred to in book.) ] 85. NON-REFRACTION AND BUT LITTLE REFLECTION OF X-RAYS.—He employed a very powerful refracting prism made of mica and containing carbon bi-sulphide and water. The same prism refracted light but did not refract X-rays. No one would think of making prisms for examining light, of ebonite or aluminum, but he made such a prism for testing X-rays. But if there were any refraction he concluded that the refractive index could not have been more than 1.05, which may be considered as a proof that the rays cannot be refracted. He tried heavier metals, but the difficulty of arriving at any satisfactory results was due to the resistance of such metals to the transmission of the rays. Among other tests was one consisting in passing the rays through layers of powdered materials through which the rays were transmitted in the same quantity as through the same substances not powdered. It is well known that light passed into powdered transparent materials, is enormously cut off, deviated, diffused, refracted etc., in view of the innumerable small surfaces of the particles. Hence he concluded that there was little if anything in the nature of refraction or reflection of X-rays. § 146. The powdered materials employed were rock salt, and fine electrolytic and zinc dust. The shadows, both on the screen and as recorded on the photographic plate were of substantially the same shade as given by the same materials of the same thickness in the coherent state. One of the most usual ways of testing refraction of light is by means of a lens. X-rays could not be brought to a focus with the lens of what ever material it was made. Among the substances tried were ebonite and glass. As expected, therefore, the sciagraph of a round rod was darker in the middle than at the edges; and a hollow cylinder filled with a more transparent liquid showed the centre portion brighter than its edges. If one considers this observation in connection with others, namely the transparency of powders, and the state of the surface not being effective in altering the passage of the X-rays through a body, it leads to the probable conclusion that regular reflection does not exist, but that bodies behave to the X-rays as turbid media to light, § 69. 86. VELOCITY OF X-RAYS IN DIFFERENT BODIES. p. 46.—Although he performed no direct experiment in this direction yet he inferred in view of the absence of refraction at the surfaces of different media, that the rays travel with equal velocities in all bodies.