104 ^^' Brewster on some Properties of Light. 



character the images suffered only an alternate diminution of 

 brightness, in the same manner as a pencil of light receives 

 only a partial modification when reflected from water at a 

 greater or a less angle than 52" 45'. 



The different experiments which have now been mentioned 

 were repeated, with the most satisfactory results, by Mr. 

 Playfair, Dr. Hope, and Mr. John Davy. 



Although the preceding results are by no means ripe for 

 generalization, I cannot omit the present opportunity of ha- 

 zarding a few conjectures respecting the cause of this singular 

 property of the agate. 



May not the structure of this mineral be in a state of ap- 

 proach to that particular kind of crystallization which affords 

 double images ? and may not the nebulous light be an imper- 

 fect image arising from that imperfection of structure ? When 

 one of the images vanishes, the nebulous light which encircled 

 it is then a maximum, and it gradually diminishes during the 

 re-appearance of the image. When the image which had dis- 

 appeared recovers its full lustre, the surrounding nebulosity 

 is very small, and this remaining light is, in all probability, no 

 portion of the unformed image, but merely a few scattered 

 rays arising from the imperfect transparency of the mineral. 



By forming the agate into a prism, the nebulous light should 

 be separated from the image which it encloses, in proportion 

 to the angle contained by the refracting planes ; but owing, 

 perhaps, to the smallness of its double refraction, if it has 

 such a property, I have not observed any separation of this 

 kind. 



The incurvated form of the nebulous light corresponding 

 with the curvature of the laminse, seems to connect it with the 



