18 Prof. Knoblauch on the Relation of the Transmission of 



The quaUtativc cliffcreuces that were observed are exhibited in 

 the followine: table : — 



It is worthy of remark, that with the same position of the 

 plane of polarization and of the optic axes, the heat, in the case 

 of hornblende, passes worse through blue glass and better 

 through yellow glass ; and in the case of pistacitc, better through 

 blue glass and worse through yellow glass, than the rays for 

 which the planes enclose an angle of 90°. In the case of red 

 and green glass this antithesis is not observed. 



M. Knoblauch next placed two plates of pistacite together, 

 both cut at right angles to the middle line, and examined their 

 effect upon the passage of heat in the direction of the said line, 

 first, when the planes of the optic axes of both plates were 

 coincident ; and secondly, when they were at right angles to 

 each other : the proportion of the quantity transmitted were 

 as 100 : 130, the former representing the quantity transmitted 

 when the planes were at right angles to each other, and the 

 latter the quantity transmitted when they were parallel. 



Qualitative differences were also exhibited, as shown by the 

 followinc; table : — 



lu these experiments the NichoFs prism was removed, and 

 the light direct from the mirror of the heliostat was permitted 

 to pass through both plates successively. 



A plate of broicn mica was next examined, in the same manner 

 as the heavy spar, topaz and hornblende. In this case the ratio 

 of Q to Q' was as 100 : 109. 



The power of the heat to pass through glasses of different 

 colours is exhibited in the following table : — 



