1 50 Professor Forbes on the Refraction 



Mean at 0°...6°.9 

 180°. ..7 .0 



Mean, . 6 .9 ) 

 Mean at 90° and 270% .5.6) Ratio 10 ° : 81 ' or 19 P er cent polarized. 



The general concordance of these experiments will be gather- 

 ed from the following list of results. 



37. With non-luminous heat from brass about 700° ; ratio of 

 effect, when plates E and F were parallel and crossed, 100 : 78 ; 

 100 : 76 ; 1 00 : 80 * ; 100 : 81 (from five observations each), with 

 plates E and A (from three observations each), 100 : 74 ; 100 : 59 ; 

 100 : 68 ; 100 : 60 ; with A and B, ratios 100 : 78 ; 100 : 72. 



38. With non-luminous heat from mercury, about 500°, plates 

 E and F ; 100 : 77 ; 100 : 90, plates E and A ; 100 : 88 ; with 

 A and B, 100 : 78. 



39. But even with heat from water below the boiling point, 

 I was able, by the improved method of observing the galvanome- 

 ter, art. (5), (6), to establish completely the polarizing effect. One 

 series of six comparisons (conducted as in (20),) gave for the pro- 

 portions of heat transmitted, when the plates E and F were 

 parallel and crossed, 100 : 93 ; another of eight comparisons, gave 

 100 : 96 ; a third, of eight, 100 : 92. Among these twenty-two 

 comparisons, only one gave a result slightly negative. 



40. With platinum rendered incandescent by alcohol, the ef- 

 fect appears decidedly greater than with any other source of heat 

 1 have tried. Plates E and F ; ratios of effect when parallel and 

 crossed, 100 : 59 ; 100 : 62 ; 100 : 66 ; 100 : 54. The brilliancy 

 of the incandescence affects materially the transmission. 



41. Alcohol flame, as might be anticipated, is less steady; 

 means from sets of five observations, with plates E and F ; 

 100 : 66; 100 : 72; 100 : 79; 100 : 42; 100 : 62. 



42. With the simple oil-lamp of Locatelli ; plates E and 

 F, the ratios are 100 : 76 ; 100 : 73.5 ; 100 : 79. 



* Plate B was used to polarize in this experiment. 



