488 Intelligence and Miscellaneous Articles. 



(5) The interference-colour of heat can also be observed if, as in 

 the case of mica, the substance of the doubly refracting crystal has a 

 special absorptive power for certain thermal colours which increases 

 with the thickness. A new field is thus opened for thermochrose in 

 this observation. 



(6) The phenomena of heat and light obey in this the same general 

 laws. The former draw attention to facts which have hitherto been 

 little observed, or not established with equal certainty. More espe- 

 cially, in this case the deportment of the total heat, consisting of in- 

 visible and visible rays, does not correspond to that of the visible part. 



(7) The intensity of the heat which passes through two Nicol's 

 prisms, whose principal sections form an angle 0, is proportional to 

 cos 2 <p. Hence under an angle of 45° it is half as great as in the case 

 of parallel principal sections. 



(8) On interposing a doubly refracting plate between the two 

 Nicols, the intensity is 



I = z[cos 2 acos 2 (0 — a)-f sin 2 a sin 2 (<j>~— a)], 



if i represents the heat which passes when the principal sections of 

 the Nicols and the axial plane of the crystal are parallel, a the 

 angle between this plane and the principal section of the polarizing 

 Nicol, (p that between the principal sections of the two Nicol's 

 prisms. 



Among the numerous changes which here take place we must note 

 as characteristic points : — 



When the plate of crystal is fixed and the analyzing Nicol is ro- 

 tated, the intensity may considerably alter — for instance, propor- 

 tionally to cos 2 (f) when the axial plane coincides with the principal 

 section of the polarizing Nicol (a = 0) ; or it may be quite constant 

 during that rotation when the axial plane is at an angle of 45°, and 

 when at 135° to the principal section of the polarizing Nicol. 



With the Nicol fixed and the plate of crystal turned, 



(a) The principal sections being parallel, the position of the axial 

 planes at 0°, 90°, 180° produces the intensity i ; placing them at 

 22°-5, 67°'5, 112°-5, l-57°'5 ft..; while at 45° and 135° we have \i. 



{b) If the principal sections are at right angles, the first arrange- 

 ment of the crystal, at 0°, 90°, 180°, transmits no heat ; that at 22°'5, 

 67°-5, 1 12 0< 5, 157°'5 \, i; the last, at 45° and 135°, again transmits \i. 



(c) The principal sections being at 45°, when the planes of the 

 axes are at 0°, 45°, 90°, 135°, 180°, the intensity J is obtained; at 

 22°-5, 112°'5, f i i and at 67°'5 and 157°'5, \ i. 



(9) All these phenomena of the colours and intensities of interfe- 

 ring heat-rays (which hold good even in detail) confirm the assumption 

 of transverse waves of various length, by which the phenomena are 

 explained. — PoggendorfFs Annalen, May 1867. 



Liii, 



• 



