Wave-lengths in the Invisible Prismatic Spectrum. 199 



mirror of speculum metal of 1*63 metre radius of curvature, 

 and exposes a ruled surface of 129 centimetres. By this 

 large surface a spectrum is produced sufficiently hot, even in 

 its lower wave-lengths, to affect the bolometer-strips after 

 the various reflections and absorptions to which the heat is 

 necessarily subjected in passing through the apparatus. 



Fig. 1. 



