TRANSMISSION THROUGH METALS, ETC. 15 



determined within 3 or 4 mms. The interposi- 

 tion of a sheet of lead or glass 4 mms. thick 

 causes this action to disappear. By varying the 

 value of /, other values of /' 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 '94 2. 



While pursuing these experiments, I ascer- 

 tained 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, 



