400 Intelligence and Miscellaneous Articles, 



neous absorption on account of the greater thickness, held one another 

 in check, so that the action of the rays was not perceptibly changed 

 until the angle of 55° was attained, upon which there was a sudden 

 diminution of the transmission. With a uniformly darker colouring 

 of the plates, the diminution in intensity might take place before 

 that angle was attained ; yet the diminution then was never so 

 sudden and considerable. 



A corresponding deportment was observed with other bodies. 

 Gold, on account of its considerably larger angle of polarization, of 

 about 70°, was very characteristic. 



When there was no absorption, the thickness and number of plates 

 had no influence on the angle of inclination at which the transmis- 

 sion attained its maximum, as this always occurred at the polarizing 

 angle of the substance in question. Yet with absorbing layers it 

 depends on their thickness and the degree oftlieir absorption, whether, 

 and at what number of plates an increase of transmission takes place 

 at all, and whether diminution only occurs at the polarizing angle, 

 or whether, when absorption preponderates, it occurs before. 



If it be considered that, from the observations communicated, the 

 rays are the more capable of entirely or partially traversing trans- 

 parent or diathermanous plates, the more completely they are polar- 

 ized in a plane at right angles to their plane of refraction, the greater 

 the number of these plates, and the more their inclination towards 

 the rays approaches the polarizing angle — and if it be assumed that 

 refracting plates polarize heat, as well as light, more completely at 

 right angles to the plane of refraction* the greater their number 

 and the more their inclination approaches the angle in question f, it 

 will at once be seen how the row of plates put the rays which enter 

 them more and more into the condition of penetrating the succeeding 

 ones with greater facility, and how the conditions for making the 

 rays in the given case more capable of transmission coincide with 

 those under which the layers themselves opposed to these become 

 more permeable. 



As polarization by simple refraction is, in the most favourable case, 

 most nearly complete by a Nicol's prism J, it is always explicable 

 why polarization through a Nicol effects a greater increase of inten- 

 sity of the transmitted rays when its plane of polarization coincides 

 with that of the refracting piece, than does the glass piece alone. 



On the other hand, the difference between the increase on attain- 

 ing, and the diminution on exceeding the angle of polarization, must 

 be smaller in the former case than in this ; for in the former case 

 the polarization of the rays is throughout unchanged, and transmis- 

 sion is only favoured by the position of the plates ; for instance, in 

 the case of glass it is favoured between 50° and 55°, diminished from 

 55° to 60° ; in the plane piece of glass the polarization of the rays and 

 the influence of the plates on the transmission are simultaneously 

 changed ; at first both are increased, afterwards both diminished ; 

 the passage through the polarizing angles is hence the more marked. 



But among the elucidations which refer the present case to more 

 general points of view, it loses its surprising character ; the more 

 copious transmission of the thermal or luminous rays incident in the 

 naturalcondition through inclined diathermanous ortransparentplates 

 follows as a direct consequence. — Poggendorff's Annalen, May 1866. 



* Pogg. Ann, vol. cxxiv. p. 175. f Ibid. p. 176. % Ibid. p. 178. 



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