290 Mr. Talbot's Facts rclatiiig to Optical Science. 



the same position which they originally occupied. When the 

 eye looks lengthwise through a piece of doubly refracting 

 spar thus prepared (and therefore very obliquely through the 

 Canada balsam), only one image is seen. If two such instru- 

 ments are placed one before the other, and the observer looks 

 through both, then, if they are in a similar situation, a printed 

 book can be read through the combination with ease and di- 

 stinctness ; but if one of them be turned round 90°, total dark- 

 ness is the consequence*. 



Now it will be observed, that the inventor attributed the 

 fact of the instrument's producing only one image to a great 

 " divergency" which it causes in the images, throwing one of 

 them aside, out of the field of view. The German writer fol- 

 lows the same idea, but adds, that in his opinion such diver- 

 gency is caused by the Canada balsam, whose index of refrac- 

 tion being 1*549, is intermediate between that of the ordinary 

 ray l - 65l«, and that of the extraordinary ray 1 # 483, which 

 circumstance will (in his opinion) account for the rays being 

 " thrown opposite ways". He adds, that any one " who was 

 not afraid of the trouble" might easily calculate the path of 

 both rays; a remark which shows that his idea was, that they 

 were both transmitted, and diverging from each other. But 

 I find that this great divergency does not in point of fact exist; 

 for by inclining the instrument, a position may be found in 

 which both the images are seen, and they are then very little 

 separated, not more so than they were by the same piece of 

 spar before its bisection and cementation. On gradually al- 

 tering the position of the instrument, the second image is not 

 seen to move away from the first. But at a certain moment it 

 vanishes suddenly, without leaving the smallest trace of its 

 existence behind. Having thus described the appearances as 

 I have found them, I will give an explanation of them which 

 I hope will be more satisfactory. 



As long as the rays composing the two images are incident 

 upon the Canada balsam at moderate obliquities, I will ven- 

 ture to say that it cannot exert any particular discriminating 

 action upon them. But when the obliquity reaches a certain 

 point, one of the images suffers total internal reflection, be- 

 cause the Canada balsam is (with regard to that image) a less 

 refractive medium than Iceland spar. But with regard to 

 the other image it is at the same moment a more refractive 

 medium than the spar, and therefore it suffers that image to 

 pass alone. 



* This instrument is now made in considerable perfection by Mr.Watkins, 

 optician, of Charing-cross. 



