TRANSMISSION THROUGH METALS, ETC. 17 



may mention tinfoil, sheets of copper and brass 

 0*2 mm. thick, a sheet of aluminium 0*4 mm. 

 thick, a steel lamina 0*05 mm. thick, a silver 

 leaf o'i mm. thick, a paper booklet, containing 

 twenty-one gold leaves, a glass sheet, 0*1 mm. 

 thick, a sheet of mica of 0-15 mm., a plate of 

 Iceland spar of 0*4 mm., a block of paraffin of 

 i cm., a beech board i cm., a plate of ebonite of 



1 mm., etc. Fluor spar is but slightly transparent 

 with a thickness of 5 mms., similarly sulphur 



2 mms. thick, and glass i mm. thick. These 

 results I only give as a first indication, for when 

 they were obtained, the co-existence of four 

 different species of radiations, which may have 

 very different properties, was not taken into 

 account (note 5). 



It will be highly interesting to investigate 

 whether other sources, and in particular the sun, 

 do not emit analogous radiations to those we 

 are dealing with in the present communication, 

 and also whether the latter produces any calorific 

 action (note 6). 



Now, ought these radiations in reality to be 

 considered as akin to the large wave-length 

 radiations discovered by Professor Rubens? 



