determined within 3 or 4 mms. The interposition of a sheet of lead or glass 4 mms. thick causes this action to disappear. By varying the value of p, other values of p' are obtained, and substituting these values in the lens formula, the number 2.93 is obtained for the refractive index, being the mean value derived from a series of determinations as concordant as the precision of such observations could entitle one to expect. Similar experiments, made with another quartz lens, having a focal length of 33 cms. for yellow light, gave for the index the value 2.942.
While pursuing these experiments, I ascertained the existence of three other species of radiations, for which the index of quartz has values 2.62, 2.436, 2.29 respectively. These indices are all greater than 2, which explains the following fact: if in the path of the rays emerging from the lens a quartz prism of 30° refractive angle is placed, in such a way as to receive these rays in a direction sensibly normal to one of the refracting faces, no refracted pencil is obtained.
The radiations from an Auer burner,
