Intelligence and Miscellaneous Articles, 487 



ON THE INTERFERENCE-COLOURS OF RADIANT HEAT. 

 BY H. KNOBLAUCH. 



The following are the chief results of an investigation of this sub- 

 ject on which Professor Knoblauch has been engaged. 



(1) When two groups of rays meet under certain conditions, ra- 

 diant heat differs in its properties after the meeting ; for instance, as 

 regards its property of traversing diathermanous bodies, it manifests 

 an interference-colour. 



(2) If this is produced in doubly refracting crystals placed between 

 a polarizing agent (for example two Nicol's prisms), when the plate of 

 crystal is fixed and one Nicol is rotated, the colour passes through 

 white to the complementary. 



(3) The maximum of these differences ensues if the polarizing ar- 

 rangements are parallel in one case, and in the other make with each 

 other an angle of 90°, — the plate of crystal being fixed, for the most 

 favourable case (that is, in the case of mica) at an angle of 45° be- 

 tween the original plane of polarization and that of the optical axis 

 of the mica. The passage through absolute colourlessness takes 

 place when the polarizing plates form an angle of 45°. That special 

 shade of colour which, when the angular distance is again halved, 

 occurs at 22° # 5 in the case of one and at 67°*5 in the case of the 

 other colour, is almost exactly halfway between the full degree of 

 colour and absolute want of it, being, how r ever, somewhat nearer the 

 latter. When the rotation of one polarizing arrangement is from 90° 

 to 180°, the complementary colour in a corresponding manner passes 

 through want of colour at 135°, again into the original colour. 



(4) On rotating the plate of crystal in its plane, w r hen the prin- 

 cipal sections of the Nicols are parallel, only one thermal colour 

 occurs ; when they are at right angles, the complementary colour ; 

 while when they form an angle of 45°, both thermal colours appear. 



(a) In the first case the complete colour appears when the plane 

 of the axes is arranged at 45° and 135° to the principal sections of 

 the Nicols, and colourlessness at 0° (that is, when the three planes 

 coincide), at 90°, and 1 80°. The shade of colour which occurs at the 

 intermediate angles 22 0, 5, 67°*5, 112°'5, and 15/°*5 is much nearer 

 the point of colourlessness than the transition colour on rotating the 

 polarizing arrangement. 



(b) In the. second case the full degree of saturation of the comple- 

 mentary colour is unaltered. The colour is only interrupted by the 

 disappearance of the rays themselves, which ensues when the plane 

 of the axes of the crystal is arranged at 0°, 90°, and 180°. 



(c) In the third case the thermal colour mentioned under («) 

 occurs at its maximum when these planes are at 67°*5 and 157°'5. 

 The complementary colour at 22°*5 and 1 12°'5 is observed in only 

 one shade of colour, which is just as near colourlessness as that 

 shade of the first colour mentioned in (a), and which occurs at 22 0, 5, 

 67°*5, 112 0, 5, and 157°*5. Colourlessness occurs when the crystals 

 are in the positions 0°, 45°, 90°, 135°, and 180°. 



